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Chapter 3 Putting electricity to use. Electric motors and telegraph were the first wide spread use of electricity. Faraday produced a crude electric motor (1821) before William Sturgeon demonstrated the first efficient electromagnet (1825) and well before Samuel Morse developed the first telegraph (1837). For technical reasons that will become apparent we will look first at the electromagnet, then the telegraph and finally the electric motor. Each of these inventions had far reaching effects on the development of electricity and electronics. Electromagnets are still used today in solenoid switches and the like. From early work with these devices sprang the idea for telegraphs. This in turn led directly to telephones and more indirectly to radio. Electric motors have become so ubiquitous that you probably don’t realise how many of them surround you. Washing machines may immediately come to mind but what about your printer or CD player or microwave oven or even your mobile phone (an electric motor makes them vibrate when a call comes in). Electrical pioneers like Faraday and Oersted probably had little idea how much their efforts would shape the future. Electromagnets. These devices are generally quite simple in operation. If current is supplied to a wire coiled about a bar of iron a magnetic field will be generated around this device. We made a crude electromagnet using a bolt and enamelled copper wire. This wire has a flexible coating of insulation around it and is used to make generators, motors, solenoids and many other devices. The coating can easily be scraped away exposing copper wire to form electrical connections. You can buy a roll of enamelled copper wire for a few dollars from electronic hobby stores like Radio Shack. Thin wire with plastic insulation will also work in this instance. Uninsulated wire is no good because as soon as conductive coils of metal touch electricity will jump from one coil to another instead of going right the way around in a loop. Electricity will always take the shortest path available. We used about 100 turns of wire around the bolt and a 9-volt battery to power the electromagnet. If you decide to make an electromagnet like the one pictured it is important to only connect it to a small battery (9 volts is about right). Too big a battery (e.g. a car battery) can supply far too much current causing the wire to heat up rapidly and quite probably melt. Please don’t even think of connecting your electromagnet (or any other DC device) to mains electricity, as this is very dangerous. The first Telegraph. Our electromagnet simply picked up a few paper clips but more efficient ones can do all sorts of useful things. They can operate a valve like in garden reticulation systems or can open (or close) a switch like the relays in your car. They can ring a bell like the one illustrated. When the switch is closed current flows through the electromagnet’s coils. A magnetic field is generated pulling the iron armature towards the electromagnet. A hammer attached to the armature will then strike the bell. This was the very system used to demonstrate the first practical telegraph. The only real difference being that battery and switch was approximately one mile away from the electromagnet and bell. Samuel Morse then set about developing a code that could easily be deciphered by an operator listening to the bell. Hence Morse code was born. The bell was later replaced by a pen that wrote upon a length of paper drawn by an electric motor. Operators quickly learnt to recognise code from the tapping noises made by the pen. This led to a simplified system whereby the pen and paper were deleted leaving just an armature to tap out its noisy code. Electric motors. Lets begin by imagining an electromagnet mounted on a spindle, this arrangement is known as an armature. A permanent magnet is mounted directly below the armature. We won’t worry about how the electromagnet is powered at this stage. While there is no current supplied to the armature it will rest in whatever position it was left in. Once current is supplied a magnetic field is generated which will be attracted to the permanent magnet. The armature will turn on its spindle to align with the magnet. Once aligned the attraction between armature and magnet will prevent any further rotation. If the armature is to keep rotating then current will need to be switched off just as the armature and permanent become aligned. By switching the current on and off at just the right time our armature will remain spinning. A device called a commutator achieves this effect. Simple motor. We built a simple electric motor from enamelled wire and a permanent magnet (see photo). The wire was wound twenty times around a broom handle to form a coil leaving a long strand of wire at each end. One strand was stripped of insulating enamel (an old hacksaw blade does a good job) while the other only had enamel removed from one side along its length. Half of the strand was exposed and half insulated. The coil was then mounted in a crude wooden cradle. Two lengths of tie wire (any stiff uninsulated wire will do) were then screwed to the base and bent so that they rubbed against the outstretched strands of enamelled wire. These tie wires now form what is known as brushes whose purpose is to supply electric current to the coil. As one of the coils connecting strands has only had half its insulation stripped away electric current only flows through the coil during half of its rotation. Once the coil starts to spin an electric current is continuously switched on and off causing a magnetic field to also be created then dissipated around the coil. This field is attracted by the permanent magnet keeping our simple motor spinning happily. Motor with commutator. The second motor we built had four electromagnets mounted on a wooden block. These consisted of one hundred turns of enamelled wire wrapped around steel wood screws. Long strands of enamelled wire were left on each winding (see picture). Two nails were inserted into the block to form a spindle. A short piece of wooden dowel was mounted on one of the nails. A single strand from each of the coils was stripped of enamel and attached to one of the nails. The four remaining strands were also stripped of enamel and laid flat upon the wooden dowel. The armature was then mounted in a wooden cradle and contact wires (brushes) mounted on the base just like the simple motor’s brushes were mounted. One brush contacted the wires mounted on the wooden dowel while another was in contact with a nail. We’ve drawn this armature with just one winding to show how current flows through the armature. Starting at the left hand contact wire current flows up to and through the left hand nail, through windings (coil of wire) and onto wire attached to wooden dowel. The wooden dowel and wires are called a commutator, its purpose is to connect and disconnect coils of an electromagnet at just the right time to keep the armature rotating. From here current will travel down the right hand contact wire and back to its source (e.g. battery). If a permanent magnet is strategically placed the coils magnetic field will be drawn towards it. Here are some small photo’s of the motor described above. The commutator in our motor switches each coil on then off again as coil and magnet become aligned. By doing so the motors armature will keep spinning as long as sufficient current is supplied to the contact wires. A word of caution to those who wish to build this motor. A 12 volt battery is required and the motor windings can get very hot particularly if the armature is not turning. Leaving a battery connected to motor for too long may cause it to catch fire. Dynamo’s reviseted If we were to replace the battery running our motor with a volt meter then spun the armature by hand we would see a voltage produced. Most permanent magnet type electric motors will act as generators when spun by hand or driven by another source. When we looked at a simple dynamo in chapter 2 we found that it produced alternating current. That is to say current flowed in one direction as the magnet and coil approached each other and in the opposite direction as magnet and coil moved apart. The early developers of electric devices had difficulty useing alternating current (AC) so a means of switching off current flow as magnet passes coil needed to be found. It was soon found that a commutator can do just this (see illustration) so that a source of direct current (DC) could be mechanically generated. The current wasn’t smoothe like that of a battery (it pulsated with each pass between coil and magnet) but at least it didn’t change direction. The only difference between our motor and generator is that the motor was connected to a battery and generator connected to a volt meter. Since the development of small, cheap and reliable rectifiers DC (direct current) generators are gradually becoming obsolete in favour of more efficient alternators. Where DC is required we use various forms of rectifier to convert AC to DC power supply. We will be looking at how rectifiers work in chapter 5. Most electricity is produced by alternators these days, our domestic mains power is AC (alternating current) and home appliences are especially designed to work with AC. Up until the end of 1960’s car electrical systems were powered by a DC generator while the engine was running. Once high current solid state rectifiers became available automotive AC alternators quickly replaced the less efficient DC generators. Telephones. Many people know that Alexander Graham Bell was the first to patent a telephone in 1876. I wonder how many know that he didn’t set out to develop a telephone. Mr. Bell was actually trying to increase the capacity of telegraph systems by using oscillating (vibrating) electrical signals of various frequencies. He set up a mini telegraph system between two rooms to test his ideas when he realised that his ‘telegraph’ was transmitting the sounds of a clock ticking away in the other room. Thus the telephone was born all be it somewhat serendipitously. He was also fortunate to be credited with the telephone’s invention as his patent application was filed only hours before a rival applied to patent his system. We’ve allready looked at the electrical principles that led to the development of telephones. If a magnet is passed by a coil of wire then an electric current becomes available in that wire. We call this process magnetic induction and say that the magnet induces an electrical current in the coil of wire. Lets mount a small magnet inside a coil of wire and then attach that magnet to a flexible diaphragm. If the diaphragm vibrates then the magnet will vibrate with it. As the magnet moves within the coil of wire so a small electrical current will be produced (induced). This current will change direction as the magnet changes direction and will increase in intensity as the magnets movements also increase. The electrical current will in many ways mirror the diaphragm’s movements. If sound waves cause the diaphragm to vibrate then the magnet will induce an electrical signal into the coil that mirrors the sound . We have just described the workings of a crude dynamic microphone. Electrical signals can also be turned back into sound by way of a speaker. These work in the opposite way to a microphone. Mechanically they are very similar in that a magnet is attached to a diaphragm (a much larger one known as a speaker cone) and a coil of wire surrounds the magnet. Those of you with knowledge of speakers will know that they are actually arranged differently though the principle is exactly the same. When the coil has an electrical signal applied it pulls on the magnet causing a vibration to occur in the speaker cone. If we attach the coils of a microphone to a distant speaker’s coil by way of wires then we have constructed a simple telephone. We can follow the sequence in our illustration. Sound from the violin makes the diaphragm vibrate. The magnet being attached will vibrate with the diaphragm inducing an electrical signal into the coil that surrounds it. This signal will travel down the wires into the speakers coil thus tugging on a magnet attached to the speaker cone causing it to vibrate just as the microphone’s diaphragm did. Our violinist’s music will then be audible from the speaker. Speakers can be used as microphones though microphones don’t make very good speakers (their diaphragm is too small for one thing). Modern telephones are of course more sophisticated than this, they have a dialler, an amplifier, a special coil to stop your own voice being audible in the earpiece and plenty of other electronic tricks to make them as functional as they are. The basic principle of turning a sound into an electrical signal and back into a sound at the other end is still the same though. Electric lighting. Early reserchers into electrical devices soon found that passing too much electrical current down a wire will cause that wire to get hot and if enough current is available the wire will melt. It didn’t take long for them to realise that if they could get a wire to glow bright enough then a source of electric lighting would be possible. There were obstacles to over come though. Firstly there needed to be found a material that would conduct electricity and be able to glow bright enough without melting. There was also the problem of oxidation, suitable materials that got to white heat invariably burned away quickly. No one realy agrees as to who invented the electric light, Humphrey Davey (UK) demonstrated both incandescent and arc lights around 1800 to 1804. Most early electric lights were arc lamps which produced light by creating a spark between two carbon rods. These lights were inefficient (used lots of electricity), were too bright for many applications and tended to flicker unevenly. Incandescent lamps (with glowing wires or filaments) were attempted by many reserchers from 1802 onwards but it wasn’t untill between 1877 and 1878 that Joseph Swan (UK) and Thomas Alva Edison (USA) both pioneered the use of durable carbon filament incandescent lights. The first successful light globes consisted of a bulbous tube of glass within which was mounted two wires. Between the wires there hung a thin filament of carbonised cotton. The glass tube was evacuated of air and sealed with the two wires ends protruding outside. Electrical current was then fed to the two wires causing the fillament to glow white hot. About 1910 carbon filaments were replaced by tungsten ones and light bulbs were filled with the inert gas argon instead of being evacuated. The modern electric light had arrived and has changed little since. During the early proliferation of electric lighting a fierce debate errupted as to whether they should be powered by AC or DC electric current. Edison stoically led the campaign for DC but lost out to his former emploee and fierce rival Nicola Tesla for a simple technical reason. The only way to efficiently send electricity down very long wires is to do so at high voltages. AC current can be raised or lowered in voltage by devices called transformers alowing huge voltages to be sent down transmission lines yet much safer voltages alowed into homes etc. Transformers don’t work with DC current so Edison’s system was doomed from the start. Transformers Lucien Gaulard and John Gibbs demonstrated a transformer in London during 1881 although it was Nicola Tesla who developed the first efficient transformers a few years later. If two coils of wire are wrapped arround the same iron bar and one of those coils is supplied with AC voltage then another voltage will be created in the second coil. As voltage increases in coil 1 then the iron bar will become an electromagnet whose strength and polarity pulsates along with the input voltage. This pulsating magnetic field then creates a voltage in coil 2. The correct term for this process is induction, we say that a voltage is induced in coil 2. When both coils have the same number of turns then the votage induced in coil 2 would be the same as that supplied to coil 1. Should coil 2 have twice as many turns as coil 1 then the voltage induced is also doubled. Of course this doubling of voltage does not increase the total amount of power available, you can’t create energy from nothing, when voltage is doubled current is at least halved. You may recall the formula we presented in chapter 2; Power(in Watts) = Amps x Volts. Power availbale at the transformers secondary cannot exceed that supplied to its primary windings. The ratio of turns between coil 1 (primary coil) and coil two (secondary coil) will be the same as the ratio of voltage between supply and output of transformer.. Differences in current between primary and secondary also follow the same ratio but is inverted, i.e. if voltage is trebled then output current becomes one third of input current. If coil 2 has less turns than coil 1 then voltage will be reduced and current increased Although modern transformers are very efficient they do lose a little of the power supplied to the primary coil. Manufacturers allow for this and add a few turns to one or other of the coils to ensure the correct voltage is available at the secondary or output coil. Transformers that are used to increase voltage are called ‘step up transformers’ and those used to reduce voltage are called ‘step down transformers’. Radio The invention of radio owes much to the development of transformers, telegraph and telephones. The first radio transmitters were called radiotelegraphy transmitters and shared many of telegraph’s components. The story of radio’s genesis is both convoluted and controversial with a number of people making crucial contributions. In 1864 a brilliant Scottish mathematician called James Clerk Maxwell predicted the existence and nature of radio waves. His work was largely missunderstood so it wasn’t untill about three decades later that the first radio’s were developed. Even to this day there is considerable debate as to who actually invented radio. Guglielmo Marconi is generally credited with radio’s invention although in 1933 a US supreme court decision revoked Marconi’s patents in favour of Nikola Tesla. Despite this there is no doubt that Marconi achieved a number of milestones in the development of radio including the first trans atlantic radio message. The German physicist Heinrich Hertz could also be credited with radio’s invention and was the first to actually demonstrate the existence of radio waves in 1893. Around about two years later Marconi unvieled a practical radiotelegraph setting in motion a revolution that led to radio, radar, television, mobile phones and many other icons of our technological age. Early radio transmitters were rudimentary devices that created a spark between two small metal spheres one of which was connected to a length of wire called an antenna. Looking at the illustration we can follow the sequence of events. A telegraphers switch briefly supplies current to the primary windings of a transformer. The sudden surge in voltage is multiplied in the transformers secondary winding causing a strong spark to jump between two closely mounted metal spheres. A long piece of wire called an antenna would then pulse with electric charge and in doing so emmit radio waves. Recievers at this time employed an even cruder means of detecting radio waves. It was known that dust or iron filings will tend to clump together (cohere) in the presence of radio waves. Iron filings are normally poor conductors of electricity but when they clump together or cohere they conduct electricty readily. The first radio recievers were based on the coherer, a glass tube containing iron filings and sealed by a metal cap at each end. Refering to the diagram, a battery is connected to one side of the coherer while the other side is connected to an electromagnet. The electromagnet in turn is also connected to the battery. When no radio waves are present very little current passes through the coherer so the electromagnet does not attract the armature. As soon as a burst of radio waves passed over the coherer its iron filings would cohere allowing current to flow thus activating the electromagnet. One giant shortfall of a coherer was that once the iron filings had clumped together the tube had be tapped to dispurse them again hence this system was only good for the transmission of morse code. Coherers formed the basis of radio recievers for about the first ten years of their production limiting the usefullness of this technology. It wasn’t untill the invention of tuned oscillating circuits and electronic components like vacuum tubes that truly modern radio was born. We will take another look at radio in chapter 5.