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JUNE 1938 ELECTRICAL PHENOMENA IN THE POSITIVE COLUMN AT LOW PRESSURE is more advantageous to bring the atom from the 4.86 volt state, to which it has fallen after emission of the blue line, immediately back again to the 7.69 volt state, than to wait until the atom has returned to the zero state and emitted the ultraviolet line 2537 A. In the first case only 7.69 - 4.86 = 2.83 volts are needed and this energy is radiated entirely in the visible region; in the second case, however, 7.69 volts are needed ofwhich only 2.83 volts are transformed into visible light. These cumulative effects are very much favoured in the case of mercury by the fact that 4.86 volts is a "resonance level" and 5.43 volts a so-called "metastabie level". Since line 2537 is a resonance line, it is readily absorbed by' the atoms, in the . zero-state. After about 10-7 sec emission again takes place, and the energy radiated can then' be absorbed by another atom in the zero state. This process is repeated very' many times before the resonance radiation leaves the tube, so that the concentration of atoms in the 4.86 volt state ID- 165 creases sharply and with it the chance of cumulative excitation from this level to a higher one. For the metastable 5.43 volt level also the chance of cumulative excitation is large. If the atom has been brought into the 5.43 volt level by collision with an electron, it cannot easily return directly to the zero state byemission of radiation, it usually does so by means of collision with other atoms or electrons. The life of such a metastable atom, about 10-4 sec., is much longer than that of an atom in an ordinary excited state, so that the chance of cumulative excitation is thereby increased. In the low pressure mercury discharge used. in the blue illuminated advertising signs, concentration of the current therefore is an advantage. In the high pressure mercury lamps this phenomenon is even more stimulated by the compression 'of the dis.charge to a narrow zone near the axis, .' Fig. 11 gives a schematic survey of the various processes here discussed which occur in the positive column. THERMOJUNCTIONS by J. W. L. KÖHLER., 621.317.7.082.62 The construction and action of thermojunctions are discussed in this article. A detailed accoun~is given of the factors which affect the sensitivity and the characteristic of such junctions. The choice of the meter movement and the classification of thermojunctions are explained. Finally a survey is given of Philips thermojunctions. Introduction article we shall discuss the last type of instruments. When an alternating current of very low frequency is sent through a suspended coil galvanometer which has its zero point in the middle of the scale, the meter indicates the current at every moment; the pointer moves back and forth with the frequency of the alternating current. If the frequency is raised, then after a moment of resonance the deflection becomes smaller and' the pointer finally remains . practically still due to the fact that the moving . system of the 'meter cannot follow the high frequency. A .suspended coil galvanometer is therefore not suitable for measuring alternating currents. Such currents can only be measured with an .Instrument whose deflections are always in one, direction, no matter what the direction of the current, and whose pointer can therefore adjust itself to definite average deflection. This condition is fulfilled by dynamometers, soft-iron meters, rectifying meters and thermal instruments. In this t a In thermal instruments use is made of the heat development in a resistance, when a current passes through it. Etis obvious therefore that the direction of the current can have no influence on the deflection of the meter. The heat developed can be used in various ways: . i). The thermal expansion of the resistance filament can be indicated by a pointer in some way or other . b) The heat developed can be used to heat a quantity of gas whose increase in volume is indicated by a drop of liquid in a capillary. c) The change in resistance of a filament forming part of a Wheatstone bridge may .be measured. d) The heating of the filament can be measured with a thermocouple connected to a direct current meter. We shall treat the last method in detail, 166 Vol. 3, No. 6 PHILIPS TECHNICAL REVIEW Construction and action of thermo-electric ammeters KIe m en cic was the first to construct a measuring instrument on this principle. The'instrument consists of two wires of diffe~ent materials which form a thermo-electric couple, The two wires are wound together, knotted together or welded together at the middle (seejig. 1). The current to be measured Fig. 1. Kl emenë ië 's thermocouple. The wire of one metal is fastened to terminals 1 and 4, that of the other metal to terminals 2 and 3. The wires are joinedin the middle electrically in some way. The current to be measured passes through along the terminals 1 and 2, the galvanometer is connected between 3 and 4. is supplied to the terminals 1 and. 2; the wires bèè~me warm" at the' middle due to the heat development; this causes a thermal electromotive force between the terminals 3 and 4. This voltage can be measured by' connecting a sensitive meter to the terminals. There are various objections to this construction which will appear later. Fig.2 gives a sketch of Fig. 2. Modern thermocouple. Between terminals 1 and 2 a filament is stretched, and between 3 and 4 there is a thermocouple whosejunction is fastened to the fila~ent. electric ammeter. If. a current i is sent through the filament with the resistance Rg, an amount of heat equal to i2Rgt will he developed in the filament. ,The temperature of the filament will _'thereby be raised; the heat is lost by conduction over the terminals, and by conversion into electrical energy through the thermocouple and by radiation. If the amount of heat which is given up is proportional to the increase in temperature, the latter will be proportional to the heat developed, i;e. to i2• The same is true of the temperature of the junction of the thermocouple. If the thermal electromotive force is proportional to the temperature difference, it will also be proportional to i2, and the same is true of the current in the circuit comprising thermocouple and galvanometer, which current' is proportional to the thermal electromotive force. The deflection ofthe galvanometer is therefore proportional to the square of the current in the filament. The ratio between the current through the galvanometer and the current through the filament is called the characteristic of the thermocouple. Here the characteristic ' is quadratic. Therefore, with a meter having a quadratic scale division, after calibration of" one point on the scale, the primary current may be read off directly. A meter with a linear scale can àlso be used, the reading is then proportional to i2,' i..e. to the cnergy which can be developed by the prim~ry current in a resistance; this method of measurement has many practic~l applications. Mathematical analysis We shall give a brief account of the mathemaa modern thermo-electric ammeter from which it tical analysis of the action of' the thermo.'may be seen that the principle is the same as that ammeter. We shall first consider 'the filament of the original. A filament is stretched between alone without the thermocouple, The following 1 and 2. Midway between the terminals a thermodifferential. equation holds for the teinperature couple is fastened -to the filament with its junction increase in.a straight wire stretched between two on or close to the filament. The ends of the thermocouple are fastened to the terminals 3 and 4, ' points and heated by a current: between which the direct current ammeter is d21' au , Ai2(! = 0 . (1) connected. The whole is often placed in an evacuated dx2 Aq Aq2 bulb. When this is done there is no loss of heat Where: by convection, and the sensitivity of the instrument is increased. In certain types the thermocouple is x the coordinate of length of the wire from the middle in centimetres, electrically connected with the filament, in others r the tempera~ure difference with respect to the it is insulated from ,the filament. This construction surroundings at the point x in degrees centiis not so simple as the original one, but it has grade, certain advantages. One of these is immediately u. the circumference of the wire in centimetres, . obvious. 'The thermo-electric properties of the q the area of the cross section of the wire in square filament material need not be considered, so rhat centimetres, the choice becomes muclî greater. i the electric current in amperes, We shall now discuss the action' of'.the thermo- - - - •+- - I THERMOJUNCTIONS JUNE 1938 A the electrical equivalent of heat, 0.2388 calories per watt second, e the ,specific resistance of the wire in ohm centimetres, a the heat radiation in calories per centimetre second, A 'the specific conductivity for heat, in cal/cm sec. degr. C. 240 T't' , (dor) and ( -d.) . By heat dx2 dXl t . A~ [( :) 1- Ai2e An amount of heat -q ( :) J dx . ' dx is developed in the element by / ,120 ,I , 80 40 à ;;/ - <, / N. J /"" - r-, V - "'- 0,4 "'- ~ \ " '\ ~' I{'_ 0.5 0,3 G,2 0,1 0 0,1 02 0,3 x . = Aq :: ;I" 160 -T~ conduction' therefore the following amount of energy is supplied to the element: . I- .", 200 Equation (1) can be derived as follows. Consider a length dx of the wire; at the ends of this element the temperature gradients are 167 0,4 '~+T .e66S' os x Fig. 3) a) Temperature of a stretched wire through which current is flowing, . b) Temperature of the same wire when a thermocouple is fastened at its middle point. with the diameter d. The increase of t~mperature is therefore actually proportional to i2• When K is small (this is usually the case in practical appliéations as we, shall see later), we may write: . the current ·i. If the element is situated in a vacuum it loses energy to the surroundings only by radiation; the amount of this radiation is u a • dx, where it is assumed that the radiation is proportional to the temperature difference, which is permissible when the' increases in temperature are small cosh K = 1 '1(2/2, (see below for more detailed explanation), In the stationary . state the energy supplied must equal the energy given off, and (3) becomes: and therefore d2• Ai2e . Ai2 A i2 ell Aq-dx dx = uo r de, ----= dx2 q 8 .q Aq + + ~- 8 e,»; (3a) from which (1) f1llows. In this derivation it is therefore assumed that e, A and a are independent of the temperature; we shall see later what changes are necessary when this condition is no longer satisfied, In this expression Rg is the electrical and Rw the heat resistance of the filament. In (3~) the radiation constant 6 no longer occurs due to the approximation. This fact will be found later to be very imIf the limiting condition is introduced that the portant. increase. of temperature at the terminals is> zero We, shall now pass on to the cas~ where a 'thermo, . (x = l/2.), then the equation has the following couple, is fastened to the middle of the filament. solution: This can be described in good. approximation by calculating that a quantity of heat M is removed' " x ~u -. cosh at the middle of the filament per second and per' , A Ai2e 1":"" q ï:=--. (2) degree increase of temperature. The differential auq equation then becomes more' complicated; we shall cosh- l~u 2 Aq , only give the result for the increase in temperature tM at the middle of the wire: (cosh a' is the, hyperbolic cosine of a, i.e. the \ abreviated notation ofl/2.(ea e-à); the expression .1 .hyperbolic tangent which occurs later' stands for 1- --=--= Ai2e cosh K t ea - e-a· . (4). , ). In fig. 3a this increase of temperature is MtghK «v« M tghK ea e-a 1 1 ----:== drawn for' different points of the filament. 2}aAuq 2VaAuq At the middle the increase in temperature (t) is: ± + + A t=-- l i2e 1 auq where K ~~ l~u 2 - . auq V;; the fact that the heat lost flows to the middle of the :filament from parts lying near the middle. For small values of K the following holds: { 1- , Aq = 1 + The increase of temperature is therefore greater, 1. ~(3) tanhK cosh K . the smaller M. The factor takes account of =-Ai2e 1 cosh- l + ,--;;-- for round wires ... 'YaÀuq . PHILIPS 168 (4a) In this case also the increase in temperature is proportional to i2• The influence of the heat conduction is less than in (3a). This is clear enough. In the first case a great increase in temperature could be attained by a high heat resistance of the filament; the supply of heat to the middle of the filament has now become an important factor. In fig. 3b we gïve an idea of the variation of temperature along the filament; the middle no longer has the highest temperature. . . Finally the value of M must be expressed in terms of the dimensions and constants of the thermocouple. An analogous differential equation is valid here, but with different limiting conditions .. The solution is the following: _ M 1- YGlÀlUlql , tghKl Vol. 3, No. 6 TECHNICAL REVIEW (5) . with a given current. The voltage drop caused by the filament, i Rg = V, is not dependent on the dimènsions of the filament. lf a definite rise in temperature must be attained with different currents, the resist~nce of' the filament must be inversely proportional to the current. The energy necessary for a given temperature increase, V2/Rg' is inversely proportional to the resistance, and therefore proportional to the current; thermoammeters for small currents, therefore, possess a greater energy sensitivity than those for heavy currents. The foregoing remains true when the second term in the denominator is small with respect to one; this is the case when there is only a slight loss of heat through the thermocouple, or when 1/2 Î. q is small. The latter is always the case with. thermc-ammeters for larger currents (i I/q must always he constant). If the second term may not be neglected, which is the case with thermc-ammeters for low currents, the sensitivity to energy is smaller. A small heat loss through the thermocouple is therefore favourable in such a case. Since ,.1,1 and ,.1,2 are usually given (the choice of material is determined by the thermo-electric properties), this can be achieved by making ql/Il and q2/12 small. The . (!lIl (! I resistance of the thermo-couple Rc = -2 -222, however, . ql q2 becomes large as a consequence of. this measure; a limit is' thereby set to the reduction of 1\-f (see later). The dimensions of the thermocouple occur only in the form q/I, so that wires with the same electrical resistance cause the same heat loss. In the above it is always assumed that the radiation may be neglected; this assumption is the basis of the approximation employ~d here, + o where the subscript are for one of the The same formula for the other wire; 4 12 1 indicates that the values wires of the thermocouple. with the subscript 2 holds the length of the wires is 11yGlUl - and -; Kl = -- ...The loss ofheat through 2 2 . 2 Àl ql . the whole thermocouple is: +M M = MI At small values Of Kl 2• With a given thermoelectric combination the above considerations are also valid for the thermal electromotive force E'h. The sensitivity of the thermo-electro~eter is of course proportional to the thermoelectric force per degree. The current through the galvanometer im is the following, when- Rm is the resistance of the meter: (5) becomes: . t M = 2Àlql 1 ·'4 '(Sa) If we introduce this value of M into (4) the equation becomes: A i2 (! 12 tM= 8Àq2 (4b) --------~~~-------2 1 + -2~-q~ _À14_ql + -À - q_2 ~ Z2 m- E'h R+ Rm ' e, , where Re is the internal resistance couple. Connection of the thermocouple measuring of the thermo- instrument to the We may continue with our considerations along . two lines. a ) We may begin with a given thermocouple with resistance Rc, and ask what meter we must The temperature of the middle of the filament is therefore use in order to obtain the greatest deflection' with now expressed 'in the dimensions and constants of the wires . a definite primary current i. To' answer this we of the thermo-couple. must calculate the voltage on the. meter: Let us first consider the numerator of (4b). Leaving out the constants of the material we may, write: t M ~ 'q2 . _~_"_'i2R2 q2 s' The dimensions of the filament occur' only in the form I/q; filaments, therefore, with different dimensions but with the same resistance undergo the same increase in temperature Em = E'h Rm e; + Re Em Eth Rm/Re (6) Rm/Re 1 + Fig.4 gives a graphic representation of (6). The . l·equirement· is that the sensitivity to voltage of the meter . (i.e. the deflection at a definite' JUNE 1938 THERMOJUNCTIONS voltage on the. meter) shall be so great that the meter has the desired deflection with the voltage calculated from (6). When Rm = Re the thermo- r resistance; if the meter has a higher resistance, the maximum is shifted to the right, if the resistance is lower, it is shifted to the left. Em .É!Il..1,2 1.0 -- -- --- -- -- r-- 0,8 /. V 1-- - r 1\ -- I-- 'I'--" V 4 -I, 0,2 I! I1 0, o '0 169 l-t--I-a f. -Re 2GG!}!! 1 1 1 2 3 4 5 " Fig. 5. a) Voltage on the meter as à function of the resistance of the thermocouple at a constant thermo-EMF. 6 Eth 26698 t Fig. 4. Drop in terminal voltage of the thermocouple, when current is taken offby the meter. Em/E'h is plotted as a function of Rm/Re' When Rm = Re ,the most favourable energy transfer. takes place. couple gives off the greatest amount of energy to the meter. A meter with this resistance' therefore , deflection may be the least sensitive to energy ( . energy may be smallest); it is usually also the cheapest meter. A more sensitive meter may, however, also , he used. b) We may begin with a given meter, and ask how the thermocouple must be constructed in order to give the greatest deflection on the meter. This problem is not so simple. It is clear that with 'a given thermal electromotive force, and therefore a given teinperature of the junction, the voltage on the meter is highest when the resistance of the thermocouple is zero (see fig. Sa which is the same as fig.4, but with a different abscissa). We have, however, seen that the loss, of heat through the thermocouple becomes greater as the resistance is lowered, while the temperature increase of the junction is decreased by greater loss of heat through the thermocouple. The thermal electro-motive force is therefore diminished if the resistance of the thermocouple is reduced (see fig. Sb). The combination of these two effects is shown graphically in fig. Se, where the voltage on the meter isplotted as a function of the resistance of the thermocouple at a constant current through the filament. If we pass from a high value of the resistance to a low value, the current through the meter at fust rises because the total resistance falls, while the thermoelectromotive force changes very little; the current then falls again' because the thermo-electromotive force falls arid the total resistance no longer changes sufficiently. Fig. 5c is valid for a definite meter , -: ........ ....- ~ '. b . .-Re 2G700 , Fig. 5. b) Thermo-EMF as a function of the resistance of the 'thermocouple at a constant current through the filament. Em t c --Re 2G701 Fig. 5. c) Voltage on the meter as a function of the resistance of the thermocouple at a constant current through the filament. . .' Further discussion ofthe characteristic ofthe thermoammeter 0 In the deviation of the formulae it is assumed that (j, À and ()do not depend upon the temperature. The derivation is therefore only valid for small increases in temperature. If these increases are not small, the dependence on temperature of these quantities must be taken into account. In thàt case, however, the differential equation is insoluble, so that the, influence 'of this 'dependence must he estimated in some other way. 'I'his can he done very roughly. by first giving the constauts in the solution of the differential equation the values at room temperature, and then the values at' the temperature of the middle, of the filament; the difference between the two results indicates in any case the direction in which deviations may J;le expected. We shall' discuss this' for' the "various quantities which may.he ,depende~t on temperat~re. 0 , , Vol. 3, No. 6 PHILIPS TECHNICAL REVIEW 170 With each individual quantity it will he found that the result of its dependence on temperature is that the characteristic is no longer truly quadratic. In favourable cases the deviations due to the different quantities may compensate each other. e. The temperature coefficient of the specific resistance ,has very different values for different materials; it is for example positive and about 0.4 per cent per degree eentigrade for copper, platinum and 'iron, and practically zero for constantan and manganin. If e is positive the resistance increases with increasing current through the wire, so that with a high current relatively too much energy is developed. The deflection of the galvanometer will thus be greater than proportional to the . square of the primary current. Ä. The temperature coefficient of heat conductivity is much smaller than that of electrical resistance; it is weakly negative for the pure metals and positive for constantan and manganin. Experience has shown' that its înfluence is slight; the reason for this lies in the above-mentioned fact that a small value of Ä 0Ii. the one hand improves the heat insulation of the filament, and on' the: other hand it prevents the conduction of heat to the thermocouple. A change in Ä therefore will 'in the first approximation only influence the variation of temperature along the filament, but not the temperature öf the middle. This does not of course hold for the loss of heat through the thermocouple, which is proportional to the heat conductivity of the thermocouple. If the latter has a positive temperature coefficient the result is that the deflection increases more slowly' than proportional to the square of the, current. (J. The Stefan-Boltzmann law holds for the heat radiation of a black body. According to this iaw the amount of energy radiated per second and per square centimetre ~f the body is proportional to the fourth power of the absolute temperature. If thé" radiation takes \ place in surroundings at the temperaturè TO) the radiation is then: R = S (T4 _ T04) 2 cal' ,cm ·sec·degree4 . If the temperature differences under eonsideration are not great, this expression may be replaced by: R = S a (T - To) = S a ~. The quantity a, however, now depends upon the temperature difference, and as second app~oxjmation we may write: a a-r = ao (1 + a 1'). The constant a is approximately independent on the temperature difference, ,and between 200 C and 1000 C it is equal to 0.6 per cent per degree Centigrade. The dependence. of <5 on température results in the fact that the increase in temperature is less than the formula indicates, and therefore the deflection of the galvanometer changes less than proportional to the square of the primary current. In addition e, Ä and (J, the thermo-e.m.f. per degree, the resistance of the thermocouple and the heat resistance from the middle of the filament to the junction of the thermocouple may also depend upon the temperature. In many cases the temperature coefficient of the thermo-e.m.f, may be neglected. The resistance of the thermo- . couple' forms merely' a part of the resistance of the meter circuit, ·so that its influ~nce may he slight. It is very difficult to make an estimation of the heat resistance of the connection between :filament and. thermocouple, and of the way in which this depends upon the temperature;, its influence becomes less according as the heat resistance of the thermocouple is increased. With poorly made junctions, however, this influence may very well be feIt, so that the connection must be . very carefully made. From the above discussion it follows that . particularly the radiation can cause great deviations from the quadratic variation of the characteristic. It is therefore desirable to make the radiation as slight as possible. We have seen in the discussion of the expression for -the température at the middle of the filament that this temperature is independent of the dimensions for a given resistance of the :filament. The radiation, however, is 'proportional to the surface of the wire; to make the radiation small therefore the dimensions must be kept small. . At small values of K the radiation constant' (J no longer occurs in _the formulae. The value of K is therefore a measure of the influence of radiation. K is proportional to ljfiJ; the energy sensitivity on the other hand is proportional to ljd2• With constant energy sensitivity (ljd2 = Cl)' therefore; Ct d3/2 must be made as small as possible; d must therefore be small. The same holds for the dimensions of the thermocouple. Wires with differe~t dimensions but with ' the same electrical resistance cause the sàme loss of heat; if the dimensions, and therefore the surface, are small, the. radiation has the. least influence. Therefore short thin wires must be used to, construct thermojunctions with a quadratic characteristic. By this means the heat capacity of the thermo-couple is. decreased + ," THERMOJUNCTIONS JUNE 1938 at the same time, so that the final deflection of the meter is more quickly attained. A limit is set to the reduction of the dimensions, however, because such reduction diminishes the ability to withstand overloads. 171 for one minute. At the same time it is desirable to know the current at which the instrument can be used continuously. In practice it is then desirable to use a still lower current in order to have a margin of safety. ,.' In the case of the Philips thermo-junctions the current is indicated at which the thermoelectric force is 12 mV; in addition the maximum We have seen t.hat the highest temperature of 'current is given at which the instrument may the filament does not occur in the middle of the ,be used continuously and the current at which wire, but somewhere between the middle and the .it is quickly burnt through. We may call a ter~als. If the ~urrent through the filament is thermo-junction which gives 12 mV at 10 mA, made too high, the filament will burn through at a 10 mA instrument. those spots. It is therefore important that the It is therefore only possible to speak of a thermoratio' of this maximum temperature to the temjunction for a definite maximum current; the perature at the middle should' not be too great. current at which the full deflection of the meter A measure for this ratio, which is not easy to is obtained depends upon both thermocouple and calculate, is the ratio of the average temperature meter. of the wire vto the .temperature at the middle. For The following must also be kept in mind. Suppose . small values of K it is: that we have a certain meter and the corresponding 2M thermocouples for 10 mA and 20 mA. The resistance 2/3 - 4/15 K2 ,1 (1/6 K -13/180 K3) 'of the filaments, of these couples are respectively . raAuq . 75 and 23' ohms. In a given circuit the resistance I....!..5/12 K2 of the 10 mA couple may be too high, for instance when the couple must form part of a tuned circuit. At a given value of K (thus at ,a given value of In this case the resistance of the 20 mA couple is l;.yd) the ratio becomes greater if d is decreased; the dependence of d, however, is less, the smaller perhaps permissible. That couple may then he the value of K. Further it is clear that a thin wire used if a more sensitive meter is available which is more quickly damaged than a thick one at a gives the full deflection at 10 mA, when used with given temperature; this is' a re~son for not using the 20 mA couple. In certain cases therefore a dial instrument will not be used but 'a very sensitive , too thin a wire. With large current thermo-junctions K is so mirror galvanometer, in order to be able to use a . small. that the maximum temperature is practically . thermocouple withlowresistance for low currents also, Overloading and sensitivity + equal to the temperature -at the middle. In this Reversal effect case the permissible current is not limited by the The first thermo-junctions constructed exhibited' melting of the wire, but by the overheating of the a strong reversal effect, i.e. the deflection changed cement with which the thermocouple is fastened when the direction of the current of the primary . to the filament. it is very difficult to find a cement direct current was changed. There may be two which is attacked only at a higher temperature causes for this phenomènon: than that which injures the filament: ~) One of these may be the Peltier effect, which . The overloading of thermo-junctions for weak is the reverse of the thermo-electric effect. When currents is therefore determined by the velocity current is sent through a thermocouple the junction of evaporation or the melting point of the filament, becomes colder if, upon heating it, a current would that of those for heavy currents by the properties of the cement. In direct relation to overloading is the indication of the current for which the thermo-junction is, intended. Since the degree of attack on the filament ~670F • upon overloading depends on the length of time Fig. 6. a) Peltier effect in thermocouples. The junction during which a given current is sent through the of the couple lies on the filament and therefore wire, it is impossible to indicate a maximum perpart of the heating current passes through it. b) Reversal effect in thermocouples. The junction missible current, if the time is not also indicated. of the couple lies near the filament, but the heating A suitable measure is for example the current at current passes through a short section of one of the wires of the thermo-couple, . which. the filament remains absolutely Undamaged .... 172 PHILIPS TECHNICAL REVIEW have occurred in the same direction, and vice versa. The Peltier effect therefore occurs in the original thermo-electric ammeter by Kl eme.n i é, and also in' the case of the commonly used bridge circuits which we shall not discuss here. With modern thermocouples also the effect may appear when the thermo-junction is not electrically insulated from the filament, and when it lies exactly on the filament (see fig. 6a). In this type of construction part of the primary current also passes through. the junction of the thermocouple and gives rise to the Peltier effect. This may be avoided by placing the junction near but not on the filament. b) Even when the junction does not lie on the filament a reversing effect mayalso appear (see fig: 6b). At the point where filament and thermocouple touch each other, part ofthe primary current will always pass through part of the thermocouple. Due to the resistance of the latter a potential difference will arise which leads to a small extra current through the meter' whose direction is reversed when the direction of the primary current is reversed. In order to avoid this the thermocouple must be insulatèd from the filament; when this has been done the reversal effect will he found to have disappeared and the thermocouple can he calibrated with direct current. ó Vol. 3, No. 6 elaboration of this construction is formed by the bridge arrangements in which a number of thermocouples are joined to form a square; the disadvantage of this method is the presence of a strong Peltier effect. As was noted. above, the heat capacity also has some influence on the speed of indication; therefore small dimensions are desirable. The resistance of the thermocouple mayalso effect the speed. of 'indication because with too low resistance the damping of the 'metre is considerable. Dependence of the indication on frequency The heat development in a wire is independent of the frequency of the alternating current used for heating up to very high frequencies. At very high frequencies, however, the so-called skin effect appears, and the resistance, and therefore the heat development also, are greaterthan at low frequencies. For the thickness of wire used the influence of this effect is only appreciable at wave lengths of less than one metre, so that it need not usually be taken into account. It is quite another question with the influences of various capacities (see fig. 8). The capacity in ,,/ ;~ Sjleed of indication The ,final deflection is not rapidly reached with a thermo-ammeter. The main reason for this is that a temperatjrre equilibrium must he established in the filament and the thermocouple. Since the thermocouple itself does not carry the primary current, the heating up of the junction 'mu~t take place' from the filament by conduction, for which some time is necessary. A type of construction is possible in which the thermocouple alsó serves as filament (see fig.7); in this case the heat is developed in every, element of' volume of the thermocouple, so that only 'a small amount of heat needs to be transported by conduction, and the 'final deflection is quickly attained. An '/ F'nn -IU'" uu 26.704 Fig. a:Diagram of a thermocouple withthe capacities. . 2670S various parasitic parallel. with the filament is the cause of part of the current to be measured not passing through the filament, and the' result is that the deflection of the, metre is' too small. This . is sometimes compensated by the self-inductiön of tlfe filament. The capacity between the supply leads has the same effect. There is further capacity between the filament and the thermocouple which is connected with earth capacitatively through the galvanometer. This capacity is further increased by the fact that the leads of the filament and those of the thermocouple are close together in the pinch 0 Fig. 7. Thermocouple with direct heating. The condenser is introduced so that the direct voltage' given by the couple may not be short-circuited by the source of alternating current. The self-inductance is introduced 'to prevent the alternating . current from passing through the direct current metre. ..:liluu JUNE 1938 THERMOJUNCTIONS of the bulb and in the base. This capacity may be partially reduced by using thermocouples without bases in measurements at very high frequencies. The third kind of capacitive influence, the capacity of the leads to earth, is hereby also very much reduced. In order to overcome the last-mentioned influence it is absolutely essential that one side of the filament be earthed. The influence of these parasitic capacities becomes greater the higher the resistance of the filament. A thermocouple with a filament having a resistance of 20 ohms gives reliable indications at a wave length of 6 metres within 1 per cent; when the thermocouple is used without a base the indication is still reliable within 1 per cent at wave lengths of 3 meters. 173 d) The delicate part ofthe measuring arrangement, the thermocouple itself, is easily renewable; in this connection the quadratic, or at least reliably constant, characteristic is also important. Survey of the Philips thermojunctions The P hili ps thermo-electric meters are constructed with the above considerations in view. Fig. 9 is a photograph of a fully assembled thermocouple and jig. 10 shows the interior of such an instrument. The cover serves not only for decoration, but also for shielding the instrument from heat radiation from the surroundings. Advantages in the use of thermc-ammeters In conclusion we give the following list of the advantages connected with the use ofthermocouples for measuring alternating current. . a) The effective value of the alternating current is measured because use is made of the development of heat, and therefore of a mechanism which varies essentially with the square of the current. For this reason the measurement is very little affected by the presence of higher harmonics in the current measured, and it is not at all affected by the phase of these harmonics. This is a common property of all thermal instruments. b) Up to a very high frequency the indication is independent of the frequency of the current measured. c) The energy consumption is small compared with that of dynamometers and soft-iron instruments. Fig. 10. Interior The table gives a survey of the various properties of the instruments. With all types the temperature coefficient of the resistance of the filament is small; for most measurements it may be neglected. The indication is practically quadratic up to half the filament current, where the thermo-electric force is 12 mV; the deviation from the quadratic varia tion is 2 per cent at the most in this range, the thermo-electromotive force is about 4 mV at that current, so 'that a full deflection can still be obtained with sensitive dial instruments. The thermocouple is insulated electrically from the filament; the instruments can therefore be calibrated with direct current. Type number assembled. 12 mV EMF at approx. mA Th3 10 20 40 Th4 100 Th5 200 Thl Th2 Fig. 9. Thermocouple of a thermocouple. resistance of the filament ohm 75 23 7.5 2.2 1.1 resistance of the thermocouple 5 3 3 3 3 continuous use up to mA maximum permissible current mA 150 20 40 100 200 300 350 15 30 75 174 PHILIPS TECHNICAL REVIEW Vol. 3, No. 6 ASSEMBLY OF WIRELESS RECEIVERS The photograph shows two conveyor belts at which radio receivers are being assembled. The belts convey parts from one worker to the next. The girl takes the arriving piece of work from the belt, performs the necessary operations which consist in the mounting of parts, taken from the stock at her side, into the chassis by welding, soldering or screwing. For a smoothly running process the girl should just have finished her task when the next piece of work arrives. In this way the chassis is gradually assembled and at the end of the belt it can be mounted in its cabinet.