Download A simple device to illustrate the Einthoven triangle

Adv Physiol Educ 36: 319–324, 2012;
doi:10.1152/advan.00029.2012.
Teaching In The Laboratory
A simple device to illustrate the Einthoven triangle
Benjamin E. Jin, Heike Wulff, Jonathan H. Widdicombe, Jie Zheng, Donald M. Bers, and Jose L. Puglisi
Department of Pharmacology, University of California, Davis, California
Submitted 9 February 2012; accepted in final form 26 August 2012
General Description
Einthoven triangle; electrocardiogram; laboratory; medical education
The objectives of this laboratory class are as follows:
1. Students should learn how to determine the direction of
the dipole from the voltages in the leads.
2. Students should learn Einthoven’s law: that lead I ⫹ lead
III ⫽ lead II; thus, the deflection in one lead can be predicted
from the deflections in the other two.
3. Students should learn that the relationship still holds true
even in triangles that are not equilateral.
4. Students should learn that for Einthoven’s law to hold
true, the loop created by the dipole leads cannot be open; the
loop can be of any shape, but it must be closed.
THE FUNDAMENTAL ELECTROCARDIOGRAPHICAL CONCEPTS that Willem Einthoven developed more than a century ago are necessary for a full understanding of the heart’s electrical activity.
This is the inspiration for the laboratory we have developed.
We built a device that models an ECG, with three voltmeters
for the three leads and a battery as the heart dipole electrically
connected by a low-concentration saline solution, which mimics the electrical conductivity of our own bodies. Traditional
ECG laboratories have had students take the lead measurements and then determine the dipole position of the heart;
our developed laboratory allows students to manually
change the position of the dipole and thus predict the
measurements that will result in the three leads depending
on the position, which is a new method of educating about
the ECG. This laboratory emphasizes several objectives that
students will ultimately learn: how to determine the orientation of the dipole vector from the voltages of the leads,
Einthoven’s law (that lead I ⫹ lead III ⫽ lead II), and the
requirements for the law to hold true in a system. The
laboratory also explores a number of electrocardiography concepts that have traditionally been difficult to understand: for
instance, that minimal and maximal measurements in certain
leads depend on a specific position of the dipole and what
positions those are or how the shape of the actual loop the three
leads create affects the readings.
Address for reprint requests and other correspondence: J. L. Puglisi, Dept. of
Pharmacology, Univ. of California-Davis, 435 Health Sciences Dr., Davis, CA
95616 (e-mail: [email protected]).
The new demonstration device consists fundamentally of a
rectangular tray with an attached headboard as well as an
acrylic board with an Einthoven triangle diagram that has an
inscribed circle. The tray has four small pieces of wood
cemented to the inside edges to support an acrylic board about
half the distance from the bottom. The acrylic board has 1
AgCl electrode inserted into each vertex of the triangle, 1
electrode in the center of the triangle, and 12 electrodes evenly
distributed along the circle. The attached headboard has three
voltmeters to record the voltages in the three leads and a
battery to simulate the heart’s electrical vector. Figure 1 shows
the entire device.
This device was based on a demonstration offered to medical
students by the Department of Physiology (University of California, San Franciso, CA) when J. H. Widdicombe worked
there in the 1980s and 1990s.
Objectives
Class
Each class of ⬃30 students is divided into 6 groups with 5
students per group, and every group performs the laboratory
with 1 device. Through a guided process, every group has the
chance to independently rediscover the aforementioned principles.
Laboratory
The laboratory class starts with students taking their own
ECG. Based on these recordings, they identify the waveforms
and calculate their heart frequencies and main cardiac vector.
Next, they move to our device.
Laboratory Setup
To use this device, students place the acrylic board with the
Einthoven triangle diagram into the box, on top of the supporting blocks. The alligator clips for each voltmeter are connected
to the electrodes on the vertices of the triangle, representing the
leads of the ECG machine. The space in the box below the
acrylic board is filled with a 0.5 mM saline (NaCl) solution so
1043-4046/12 Copyright © 2012 The American Physiological Society
319
Downloaded from http://advan.physiology.org/ by 10.220.32.246 on October 18, 2016
Jin BE, Wulff H, Widdicombe JH, Zheng J, Bers DM, Puglisi JL.
A simple device to illustrate the Einthoven triangle. Adv Physiol Educ 36:
319 –324, 2012; doi:10.1152/advan.00029.2012.—The Einthoven triangle is central to the field of electrocardiography, but the concept of
cardiac vectors is often a difficult notion for students to grasp. To
illustrate this principle, we constructed a device that recreates the
conditions of an ECG reading using a battery to simulate the electrical
vector of the heart and three voltmeters for the main electrocardiographic leads. Requiring minimal construction with low cost, this
device provides hands-on practice that enables students to rediscover
the principles of the Einthoven triangle, namely, that the direction of
the cardiac dipole can be predicted from the deflections in any two
leads and that lead I ⫹ lead III ⫽ lead II independent of the position
of heart’s electrical vector. We built a total of 6 devices for classes
of 30 students and tested them in the first-year Human Physiology
course at the University of California-Davis School of Medicine.
Combined with traditional demonstrations with ECG machines,
this equipment demonstrated its ability to help medical students
obtain a solid foundation of the basic principles of electrocardiography.
Teaching In The Laboratory
320
TEACHING THE EINTHOVEN TRIANGLE
Students fill the bottom of the box with the 0.5 mM saline
solution, electrically connecting all the electrodes (the low
concentration of saline is intended to delay the polarization of
the electrodes). It is important to note that this saline solution
is what mimics the conductivity of the human body in an ECG
recording and provides the environment similar to the electrical
signal of the heart that is recorded at the surface of the body by
way of volume conduction. Students wait for the values on the
voltmeters to stabilize and then record the readings. This first
recording provides the basis for how to operate the machine.
Laboratory Procedures
that the bottom tips of the electrodes are submerged and thus
electrically connected, which mimics the electrical conductivity of our body fluids. The voltmeters on the headboard can
thus measure and display the voltage of each lead. The negative alligator clip of the battery is connected to the center
electrode of the circle, and the positive clip is connected to any
electrode on the circumference of the inscribed circle, representing the cardiac dipole. This entire setup is shown in Fig. 1.
Different positions of this electrical vector change the readings
of each lead, representing the changing vector orientation of
the heart. A diagram of these connections is shown in Fig. 2A.
Each box can be operated by five students. The laboratory
starts with the dry device, a one-liter bottle of 0.5 mM saline
solution, and an external digital voltmeter.
First, students must connect the lead wires to their corresponding terminals on the acrylic board, with the red wire
referring to the positive potential and the black wire referring
to the negative potential (Fig. 2B).
With a dry box, students attach the negative battery clip to
the center of the circle and the positive battery clip to the 180°
point on the circle. The device is turned on; as there is no
electrical connection between the electrodes, all voltmeters
should read 0 mV.
Advances in Physiology Education • doi:10.1152/advan.00029.2012 • http://advan.physiology.org
Downloaded from http://advan.physiology.org/ by 10.220.32.246 on October 18, 2016
Fig. 1. The entire apparatus in action. The acrylic board is put into the tray on
top of the supports, and all the lead wires are connected to their respective
places in the traditional equilateral Einthoven triangle orientation. The saline
solution is poured in, and the battery vector is placed into the 60° orientation.
The readings on the three voltmeters display 2.2 mV (lead I), 21.5 mV (lead
II), and 19.3 mV (lead III), respectively, verifying the Einthoven formula. RA,
right atrium; LA, left atrium; LL, left leg.
Now that the devices are set up and the students know how
to operate them, they can begin observing particular orientations of the electrical vector.
Minimal reading. Students attach the positive terminal of the
battery to the 30° point on the circle, which is in a perpendicular orientation to lead III. They discover that this orientation
gives lead III a minimal reading, ideally 0 mV, whereas the
values of lead I and lead II have deflections of the same
magnitude but opposite signs. They repeat this operation for
the other leads by placing the positive terminal of the battery to
the 150° point (perpendicular to lead II) and the 270° point
(perpendicular to lead I) on the circle. Students record the
voltmeter values for each orientation, including the sign. They
conclude that the minimal deflection in any lead is obtained
when the dipole is perpendicular to that lead.
Maximum reading. The next orientation can be observed by
placing the positive terminal of the battery to the 120° point on
the circle. The electrical vector is now parallel to lead III and
gives a maximum reading of lead III. Students repeat the
measurements at positions of 60 and 0°, when the “cardiac
vector” is parallel to leads II and I, respectively. They conclude
that the maximum deflection in any lead is obtained when the
dipole is parallel to the lead.
Verifying Einthoven’s law. Students place the battery connectors on the remaining orientations around the circle and
record the voltage values in each lead for every position. Using
all the collected data, students are asked to determine the
unifying relationship connecting the recordings in each lead;
ultimately, they (re)discover the following formula: lead I ⫹
lead III ⫽ lead II.
Verifying Einthoven’s law despite loop shape. Students can
then disprove the common misinterpretation that the Einthoven
triangle must be equilateral. They move the positive terminals
of lead II and lead III to the 120° point on the circle, so that
the triangle of the leads is not equilateral (Fig. 3). By
placing the battery connectors as they normally would on
any point on the circle, the students verify that the formula
still holds true and that a nonequilateral triangle can be an
Einthoven triangle.
Disproving Einthoven’s law for open loops. Students can
then check if the formula still holds true in open loops. They
keep the positive terminal of lead II at the 120° point and move
the positive terminal of lead III to the 60° point, so that the
triangle is open (Fig. 4A). They can verify that for any
orientation of the battery vector, with this open loop, lead I ⫹
lead III does not equal lead II. Thus, they conclude that an
Einthoven triangle must be closed.
Teaching In The Laboratory
TEACHING THE EINTHOVEN TRIANGLE
321
Einthoven quadrilateral. Students can construct an “Einthoven
quadrilateral” by closing the loop with an external voltmeter,
referred to as lead IV. Before students begin, they are asked to
hypothesize what the value of the voltage will be in lead IV.
They attach the negative terminal of lead IV to the 120° point
and the positive terminal to the 60° point, creating a closed
trapezoid shape with the four leads (Fig. 4B). Students will
realize from the vector positions that a new formula arises: lead
I ⫹ lead III ⫽ lead II ⫹ lead IV. This new position substantiates the point that the Einthoven triangle is really an
Einthoven loop: so long as the loop is closed, all of the
readings must “balance.” A triangle is just the most practical
shape, as it uses the least number of sides/leads.
Einthoven’s law and QRS progression. Students can use
their knowledge of the conduction pathway and normal ECG to
sequentially move the battery lead through a series of positions
that follow the QRS progression. This allows students to
appreciate the transitions between the Q, R, and S waves. This
also helps to solidify the more abstract Einthoven triangle
principles into the dynamic physiological context of the ECG.
For final verification, using the previously measured ECG,
students can use the formula lead I ⫹ lead III ⫽ lead II to
calculate the value of lead III from the values of lead I and lead II.
Student Evaluation
This device has been used in a laboratory for the HPH400
Human Physiology course, and the laboratory received an
average student analysis rating of 5.98/7.00, using a standard
student course evaluation grading scale, with a score of 1
meaning “poor” and a score of 7 meaning “excellent.” This
score of 5.98 is very good; in fact, compared with congruent
courses taught in the previous years but without the device we
built, this rating is 0.92 points, almost a full point, higher than
past ratings. This shows the effectiveness that the device has in
conveying ordinarily difficult ideas.
The process of self-education that this device and its laboratory presents constitutes an improvement on the conventional
teaching approach; the general consensus from students regarding the laboratory was “[v]ery helpful in lab” as well as “[v]ery
good at explaining the concepts in a clear and concise manner,”
as directly quoted from the student level-1 evaluation forms.
Discussion
Fig. 3. Diagram of the nonequilateral triangle position of leads in the laboratory, which is used to prove that the Einthoven triangle does not need to be
equilateral.
Customarily, the ECG laboratory consists of students taking
their own recordings, identifying the waveforms, and measuring
heart frequency, with little room for experimentation; our machine
provides an active learning approach by allowing students to
Advances in Physiology Education • doi:10.1152/advan.00029.2012 • http://advan.physiology.org
Downloaded from http://advan.physiology.org/ by 10.220.32.246 on October 18, 2016
Fig. 2. A: diagram of the battery vector wires attached to the acrylic board. The black (negative) wire remains in the center, whereas the red (positive) wire moves
around the circumference. B: diagram of the lead connections to the acrylic board. These leads create the Einthoven loop and mirror standard ECG connections.
Teaching In The Laboratory
322
TEACHING THE EINTHOVEN TRIANGLE
interact with this abstract representation of the heart’s electrical
activity. Students can change the vector position, measure the
values of the leads in different orientations, and finally reach the
same conclusions that Einthoven determined 100 yr ago. Thus,
these concepts are not memorized out of a textbook but are rather
synthesized through rediscovery. Additional reading (APPENDIX C),
combined with our apparatus, provides an active learning experience and ensures that the students receive a well-rounded education on Einthoven’s principles.
How to Build the Device
Start with the headboard. Using the 9 ⫻ 6-in. piece, cut three
rectangular holes (2 ⫻ 1 in.) evenly spaced and horizontally across the
board. Within each hole, attach one voltmeter such that the screens of
all three voltmeters are visible through one face of the headboard. The
back of the headboard must now be wired so that the three voltmeters
are connected to their respective 9-V batteries. Drill a hole into the
lower left of the back of the headboard, closer to the side, and screw
in the power switch. The voltmeters now must be attached to the main
power switch. Finally, drill a hole in the center of the D battery holder
APPENDIX A: MATERIALS AND HOW TO BUILD
THE DEVICE
Materials
The device was built from a 10.5 ⫻ 13 ⫻ 2-in. single-tier particleboard
letter tray, with a 9 ⫻ 6-in. headboard of the same material. Four support
particleboard pieces, 3/4-in. high, are placed along the inside of the box,
with their upper surfaces 3/4 of an inch from the bottom. A 10 ⫻ 12-in.
acrylic board, which has the illustration of the Einthoven triangle, is laid
into the box, on top of the supports. It is an equilateral triangle with an
inscribed circle. Holes are drilled in the vertices and center of the triangle
as well as every 30° along the circumference of the circle. Silver
electrodes, which have been chlorided by an immersion in sodium
hypochlorite, are glued into and through each of these holes to form the
Einthoven triangle and its inscribed circle, shown in Fig. 1. A template for
the Einthoven triangle diagram is shown in Fig. 5. Along the circumference, each electrode is labeled by its angle measured counterclockwise
from the 0° electrode.
The headboard contains three analog voltmeters placed horizontally, as they connect to and display the voltage of each lead. In the
center of the headboard is the battery, which represents the electrical
dipole of the heart. Finally, there is a switch to turn on/off the
machine. Figure 6 shows the headboard.
Table 1 shows a complete list of the materials necessary to build
this device.
Fig. 5. Image of the Einthoven Triangle printed onto the acrylic board,
displaying the 3 leads and 16 electrode locations.
Advances in Physiology Education • doi:10.1152/advan.00029.2012 • http://advan.physiology.org
Downloaded from http://advan.physiology.org/ by 10.220.32.246 on October 18, 2016
Fig. 4. A: diagram of the open loop position of leads from the laboratory, which is used to show that the Einthoven loop must be closed. If the loop is open,
then there is no defining relationship between the three recordings. B: diagram of the “Einthoven quadrilateral” constructed in the laboratory, made from the three
original leads and an additional voltmeter to close the loop. This exercise shows that the any loop that is closed will satisfy some variation of the formula lead
I ⫹ lead III ⫽ lead II.
Teaching In The Laboratory
TEACHING THE EINTHOVEN TRIANGLE
323
Finally, place the acrylic board on top of the supports inside the
box. Figure 1 shows the finished product with the connected leads and
a sample recording.
APPENDIX B: A BRIEF HISTORY OF EINTHOVEN AND THE
ECG MACHINE
and use a screw to fasten it into the lower middle of the headboard.
Now the headboard is set up (Fig. 6).
The body of the device is essentially unchanged from the original
tray. To connect the headboard to the tray, drill holes into the bottom
of the headboard and the back of the tray such that the two brackets
and four screws can connect the two parts. The brackets can be bent
slightly so that the headboard, when attached, is displayed at an angle.
Using the alligator clips, the wires, and the housing terminals,
construct the terminals for the three leads and connect them to their
corresponding voltmeters. The wire diagram is shown in Fig. 7; be
sure to use separate batteries for each voltmeter.
With the Einthoven triangle template shown in Fig. 5, print out the
image onto a sheet of photographic paper, glue it onto the acrylic
board, and then directly apply the lamination sheet on top, so that the
printout of the Einthoven triangle is clearly visible but cannot get wet
from any saline solution spill. To insert the AgCl electrodes into the
board, use the triangle template and drill holes that are just large
enough for the tips of the electrodes to squeeze in. These holes should
be placed in the center (for the negative terminal of the 1.5-V battery),
evenly spaced by 30° around the circle (for placement of the positive
terminal of the 1.5-V battery), and, finally, at the vertices of the
triangle (to represent the three leads of the Einthoven triangle that the
voltmeters attach to). Place an electrode in each of these holes. This
finishes the construction of the acrylic board.
Table 1. Complete list of materials necessary to build
the device
3 Voltmeters (Digital panel meter 9VDC, Velleman)
1 Tray (⬃10.5 ⫻ 13 ⫻ 2 in.)
1 Headboard (9 ⫻ 6 in.)
1 Battery (size D, 1.5 V, with holder)
3 Batteries (9 V, with holders)
8 Alligator clips
1 On/off switch
3 Battery clips
10 Plastic wire clips (5/8-in. square)
2 Brackets and 4 round-head screws
6 Housing terminals
1 Acrylic board
1 Sheet of photographic paper
1 Laminating sheet
16 Silver electrodes (Harting Elektronik)
4 Small congruent support pieces for the acrylic board
Advances in Physiology Education • doi:10.1152/advan.00029.2012 • http://advan.physiology.org
Downloaded from http://advan.physiology.org/ by 10.220.32.246 on October 18, 2016
Fig. 6. Front view (top) and back view (bottom) of a constructed headboard.
The three voltmeters and their respective batteries, the front battery, and the
connective wiring are all detailed.
The invention of the electrocardiograph at the beginning of the 20th
century constituted a major advance in the diagnosis of heart disease
(4). Earlier attempts made by Augustus Waller to measure the electrical activity of the heart by means of a mercury capillary electrometer were radically changed by Willem Einthoven (9). When
Einthoven introduced his machine in 1902, it was a technological
marvel, composed of many of the most recent technical advances. Its
most delicate component, the string galvanometer, was the most
sensitive electrical measurement device of the time (2); the movements of the thin filament were recorded in a photographic plate that
used a recently developed emulsion; the light source used was the
brightest option available, the carbon arc (used in lighthouses at that
time); the projecting microscope made by Zeiss had to be specially
designed in Jena, Germany; and the lenses, due to the large magnification used, needed to have minimal chromatic aberration and required a new type of glass, invented in 1886. Twenty years before the
electrocardiograph’s introduction, many of its components remained
to be invented, and its conception was almost inconceivable (2).
In addition to using the latest technology, Einthoven constructed
the machine with great ingenuity, as exemplified in the method used
to create the string galvanometer’s filament. This thin cable was
produced by shooting an arrow with its tail attached to a thread of
molten glass. The resulting filament was only 3 ␮m thick, thus
reducing the wire’s inertia and greatly increasing the galvanometer’s
sensitivity, especially compared with that of the original 20-␮m-thick
metal wire developed by Clement Ader in 1897. Another example of
Einthoven’s innovative mind was the use of telephone lines to transmit the electrical signals of a patient located a mile away to his
laboratory in Leyden, The Netherlands (1), making him not only the
father of electrocardiography but a pioneer of “telemedicine” as well.
The original apparatus in Leiden weighed 600 lb, mainly because of
the enormous electromagnet, which required a continuous-flow water
jacket to prevent it from overheating. It needed five people to operate and
occupied two rooms. Despite the early predictions that “this device will
not find any extensive use in the hospital” (9), Einthoven saw the
potential clinical use of his device. He contacted a scientific instrument
company in Munich, Germany (Edelmann), as well as the Cambridge
Scientific Instrument (CSI) Company in London, United Kingdom, to
manufacture his invention. Both companies produced the machine, but
Edelmann refused to pay royalties under the claim that they had substan-
Teaching In The Laboratory
324
TEACHING THE EINTHOVEN TRIANGLE
tially modified the design (4). During the next 20 yr, the electrocardiograph was developed and commercialized until machines that could be
moved to a patient’s bedside were available in 1920. A few years later,
electrocardiographs that could be carried instead of rolled around were
produced by the CSI Company. Although portable, they consisted of two
wooden cases weighing 50 lb each (8). In 1935, a lighter machine was
manufactured contained within a single case and weighing only 25 lb.
The appearance of these “portable” devices accelerated the spread of
electrocardiography. Further developments in the area of physics and electronics led to the miniaturization of the components and a further reduction in
the size of ECG machines, such as those in implantable cardioverterdefibrillators.
Einthoven laid down the basis of modern electrocardiography. He set
the standard derivations, the names of the waveforms (5, 6), and the
recording speed (25 mm/s) used by today’s clinicians. His vision and
conviction that his new machine would be a powerful tool to understand
the heart’s electrical activity set the stage for establishing electrocardiography as a field of practical value (7). The validity of the Einthoven
triangle theory initially created some controversy among clinicians (3),
and a variety of methods were used to validate Einthoven’s postulates
(11), such as passing electricity through a human cadaver and comparing the
measurements with theoretical predictions. Nowadays, Einthoven’s relationship is the first step in learning electrocardiography. The device presented
here is an effective alternative to the traditional teaching method to demonstrate the key concepts of Einthoven’s discoveries.
APPENDIX C: SUGGESTED READINGS
The following are suggested readings for this laboratory:
Amidon S, Amidon T. The Sublime Engine: a Biography on the
Human Heart. Emmaus, PA: Rodale, 2011.
Loe MJ, Edwards WD. A light-hearted look at a lion-hearted
organ (or, a perspective from three standard deviations beyond the
norm). Part 1 (of two parts). Cardiovasc Pathol 13: 282–292, 2004.
Loe MJ, Edwards WD. A light-hearted look at a lion-hearted
organ (or, a perspective from three standard deviations beyond the
norm). Part 2 (of 2 parts). Cardiovasc Pathol 13: 334 –340, 2004.
Sykes AH. A D Waller and the electrocardiogram, 1887. Br Med
J (Clin Res Ed) 294: 1396 –1398, 1987.
ACKNOWLEDGMENTS
The authors thank Dr. Yi-Je Chen for construction assistance and Dr. Peter
Cala for supporting this project.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the author(s).
AUTHOR CONTRIBUTIONS
Author contributions: B.E.J., J.Z., and J.L.P. performed experiments; B.E.J.
and J.L.P. prepared figures; B.E.J. and J.Z. drafted manuscript; B.E.J., H.W.,
J.H.W., J.Z., and J.L.P. edited and revised manuscript; B.E.J., H.W., J.H.W.,
and D.M.B. approved final version of manuscript; J.H.W. and J.L.P. conception and design of research.
REFERENCES
1. Barold SS. Willem Einthoven and the birth of clinical electrocardiography
a hundred years ago. Cardiac Electrophysiol Rev 7: 99 –104, 2003.
2. Burnett J. The origins of the electrocardiograph as a clinical instrument.
Med History Suppl: 53–76, 1985.
3. Butterworth JS, Thorpe JJ. On evaluating the Einthoven triangle theory.
Circulation 3: 923–925, 1951.
4. Fye WB. A history of the origin, evolution, and impact of electrocardiography. Am J Cardiol 73: 937–949, 1994.
5. Hurst JW. Naming of the waves in the ECG, with a brief account of their
genesis. Circulation 98: 1937–1942, 1998.
6. Kligfield P. Derivation of the correct waveform of the human electrocardiogram by Willem Einthoven, 1890 –1895. Cardiol J 17: 109 –113, 2010.
7. Rivera-Ruiz M, Cajavilca C, Varon J. Einthoven’s string galvanometer:
the first electrocardiograph. Tex Heart Inst J 35: 174 –178, 2008.
8. Segall HN. Introduction of electrocardiography in Canada. Can J Cardiol
3: 358 –361, 1987.
9. Sykes AH. A D Waller and the electrocardiogram, 1887. Br Med J (Clin
Res Ed) 294: 1396 –1398, 1987.
10. Wilson FN, Bryant JM, Johnston FD. On the possibility of constructing
an Einthoven triangle for a given subject. Am Heart J 37: 493–522, 1949.
11. Wilson FN, Johnston FD, Koss Mann CE. The substitution of a
tetrahedron for the Einthoven triangle. Am Heart J 33: 594 – 603, 1947.
Advances in Physiology Education • doi:10.1152/advan.00029.2012 • http://advan.physiology.org
Downloaded from http://advan.physiology.org/ by 10.220.32.246 on October 18, 2016
Fig. 7. Wire diagram of the connections between the voltmeters, switch, and battery in the headboard.