The Lecture Table Galvanometer
Thomas B. Greenslade, Jr.
Kenyon College, Gambier, Ohio 43022
My colleagues in the physics teaching profession like to debate about the validity of the ways of “getting the words and the phenomena” across to students. Some contend that the lecture is a thing of the past and that students should read the book first and then come to class to discuss it, while others would prefer to see no lectures at all. If you have lectures, then is it useful to do lecture demonstrations? The claim is made that demonstrators are, at the heart of it, showmen, and the students are merely entertained by the antics of the person in the front of the classroom. I quite enjoy doing lecture demonstrations, and have tried to make them interactive and the source of new ideas. Properly done, the demonstration can be as effective as the laboratory experience in introducing students to new phenomena and getting a deeper understanding of their earlier experiences in the physical world.
If you do lecture demonstrations, then you had better make them large enough for the student in the back row to see. If not, the good demonstrator keeps up a line of patter about the apparatus to the class that is quite analogous to how one writes an equation on the blackboard: you say what you are going to write, as you write the symbols you repeat them, and after you finish you look at it and ask the question, “and what do we have here”? In short, you spend time describing the apparatus and directing the attention of the students to the vital parts of the apparatus.
In this article, I want to concentrate on the Galvanometer, the device that tells the student that a current in an electrical circuit exists.1 Two demonstrations immediately come to mind that use galvanometers, both related to Michael Faraday’s (1791-1867) original discovery of electromagnetic induction in 1830. In one demonstration, a switch is closed in a primary circuit containing a coil and a source of EMF, and the galvanometer that is in a coaxial secondary circuit kicks over. In the second, the galvanometer is connected across a coil of wire; when a bar magnet is thrust into the center of the coil, the galvanometer needle moves either right or left depending on the polarity of the leading end of the magnet.
Later on I will discuss galvanometers that are themselves large in size, but let me start by giving a couple of examples of small instruments with scales that are blown up optically. Lecture table meters can only be made so large, and the alternative is to go small, and display the readings by projection. Fig. 1 and Fig. 2 show two galvanometers that were used in the place of slides in a lantern-slide projector.
The galvanometer in Fig. 1 that I photographed at Franklin and Marshall College in Lancaster, Pennsylvania, was made by James W. Queen of Philadelphia. An almost identical model is shown in the 1916 catalogue of L.E. Knott of Boston at a cost of $22. The glass section is designed to slide into the gate of the standard 3.25×4 inch American lantern slide projector, although it could also be used for shadow projection using a carbon arc light source. The D’Arsonval-type movement is contained in the box that extends to the side.
The projection galvanometer in Fig. 2 is in the collection of Jack Judson of San Antonio, Texas. The round label reads “Tottenham Polytechnic Physics Department.” This London institution was opened in 1897, and this was probably part of the original collection of apparatus. The projection meter is 3.5 in. in height to fit into the projection gate of British slide projectors. The galvanometer has to be used upright, for it is the Bréguet style in which a bar magnet, attached perpendicular to the needle, feels a torque when current passes through the coil. A counterweight below the magnet provides the gravitational torque that opposes the magnetic torque.
The Cenco voltmeter in Fig. 3 uses a galvanometer movement with a large resistor connected in series. This is connected in parallel series. This is connected in parallel with the circuit element across which there is a potential drop to be measured. It is far too new (!) to be in my collection of catalogues, but it probably was produced in the 1970s and was used lying flat on an overhead projector.
The larger galvanometers can be divided into three types, depending on the mechanism used to actuate the pointer.
In the Bréguet-type mechanism (Louis-François-Clement Bréguet, 1803-1883), a bar magnet is pivoted in the middle of a coil that carries the current to be measured. The little instrument in Fig. 4 shows the mechanism, with the magnet, in this case carried on a circular plate, removed and placed in front of the coil. Current through the coil produces a torque on the magnet, which tries to align itself with the axis of the coil. A gravitational counter-torque is provided by an adjustable weight below the magnet; moving this up and down lets the user change the sensitivity of the galvanometer.
The lecture-table instrument in Fig. 5, which may still in use for demonstration lectures at Cornell University, shows how this mechanism can be blown up so that a large class of students can see both the needle and the scale. This dual-range galvanometer has been given a new lease on life by having its scale repainted. This instrument can be found in the 1900 Max Kohl catalogue. It cost 45 marks (about $10); the original catalogue description noted that the coil was wound with “stout and thin wire” to get the two sensitivities.
This is the oldest of the three designs, and is the least robust. It has to be levelled carefully, and it is unusual to find one with an intact suspension. The mechanism can be seen in the example in Fig. 6, which came to the Greenslade Collection from Westminster College. A coil of fine wire is suspended between the poles of a permanent magnet, with the suspending wire acting as one lead (and it is intact in this artifact!) and a length of fine wire, coiled into a loose helix, leading downward from the bottom of the coil to complete the circuit. Current passing through the coil exerts a magnetic torque on it that is countered by the torque exerted by the twisted wire suspension. The magnetic torque is proportional to the number of turns of wire in the coil, the area of the coil and the strength of the field of the permanent magnet. It might be thought beneficial to have a physically large coil with many turns, but that would give the turning coil a large moment of inertia and the system would swing back and forth before settling down to equilibrium. One compromise is to have a long, narrow coil that would have sufficient surface area without having too much mass placed far away from the axis of rotation.
A similar lecture table galvanometer appears in the 1909 Central Scientific Company catalogue at a cost of $15.00. This example must have had some improvements, for it bears a 1911 patent date. Later catalogues describe it as “Originally designed after suggestions by Prof. R.A. Millikan, with its open construction, large size scale and large pointer, is especially well adapted for use on the lecture table. The size of the scale divisions and figures, and the size and shape of the pointer makes it easy to read the deflections at a distance. A second scale and pointer provided on the back aid the teacher in making classroom demonstrations. Thermo-electric currents are readily detected, as the maximum deflection produced by heating a single copper-iron junction is about 8 scale divisions. …Sensitivity – A deflection of one scale division (6.1 mm) on the circular scale 8.2 cm from the axis of the coil is produced by a current of 14 microamperes.”
This is the most rugged type of movement. It can be used with the galvanometer in any orientation, and obviates the necessity of levelling the instrument after placing it on the lecture table.
The D’Arsonval galvanometer movement was patented by Jacques D’Arsonval (1850-1940) and Marcel Deprez (1843-1918) in 1881. However, the practical form that we still use today was developed soon afterward by Edward Weston (1859-1936), the Anglo-American electrical scientist.2 Weston’s refinements include: (a) jeweled pivots for the swinging coil, (b), a flattened aluminum tube used as the light-weight pointer, (c) a light copper form on which the coil was wound (this also helps damp the vibrations of the pointer), (d) a permanent magnet that was aged to keep its magnetic field constant, (e) non-magnetic helical springs on both sides to provide the restoring torque, and (f) the pole pieces of the magnet curved so that the magnetic field in which the coil moved was constant no matter how it was displaced from its equilibrium position, thus allowing the scale to be linear.
Figure 7 shows the “Universal Lecture Table Galvanometer” made by the L.E. Knott Apparatus Co. of Boston, and listed in their 1916 catalogue at $35.00. This included shunts and multipliers to give the instrument potential ranges of 5, 25 and 125 V, and current ranges of 5 and 25 amperes. The basic meter movement, with a resistance of 1 Ohm, gave a deflection of one unit for a current of 2 mA. This instrument is still in occasional use in the Kenyon College lecture hall, primarily to show induced EMFs. I once used it in a far different way. After noting that the meter movement has a natural oscillation period of the order of one second, I drove it with the sinusoidal output of a low frequency generator, and was able to construct a rather nice resonance curve for it. This is a large instrument, and could be seen by a fairly good-sized group of sophomore students taking our “Oscillations and Waves” course.
The lecture table galvanometer in Fig. 8 was made by Siemens and Halske of Berlin: their logo can be seen on the scale. Here the designer has taken advantage of the fact that the galvanometer pointer can be double-ended, and the lower end can be seen by the demonstrator standing behind the apparatus at the lecture bench. (Fig 9) This galvanometer is in the Greenslade Collection in Gambier, Ohio.
The most compact lecture-table galvanometer that I have seen is the one shown in Fig. 10. Here, the D’Arsonval-type movement is horizontal, just like the standard cased meters that were used most electricity and magnetism laboratories. However, here the movement is in the center of the instrument, and the needle is bent over the edge of the scale. A second needle and scale at the back of the galvanometer are for the use of the demonstrator, who can see the movement of the needle. This could be connected to a St. Louis Motor,3 which, when the coil is spun with the fingers, acts as a generator and produces enough current to drive the galvanometer full scale. The sensitivity of 350 μA per scale division is enough to allow demonstrations with thermoelectricity to be performed in front of the class. The 1941-2 Cenco catalogue lists this galvanometer at $23.85.
On the same page in the Cenco catalogue is a lecture table galvanometer quite similar to the one shown in Fig. 11. That took me to our dining room, where this instrument resides. It was made by the firm of C.H. Stoelting of Chicago, and cost $40.00 in the 1912 catalogue. Stoelting was the result of the combination of a group of Chicago apparatus manufacturers in the last years of the 19th century.4 This is also a D’Arsonval instrument, and the catalogue copy takes pains to note that there is no suspension to break. The basic galvanometer movement requires 250 μA to move one scale division. Shunts are mounted on the board below the meter to allow measurements of up to 2.5 A and 25 A; correspondingly there are multipliers for voltage measurements of 2.5, 25 and 125 V full scale deflection.
The demonstration multimeter in Fig. 12, about 20 cm high, is really too small to be included in a paper of lecture table instruments. However, it is so unusual that I decided to end this paper with it. It has all the functions of the once popular Simpson Meter except the portability. The Simpson was the basic test instrument that could be found on every experimentalist’s lab bench in the years after 1950. The use of the 1N34 crystal diode rectifier certainly pins it down to that era. The basic galvanometer has a full-scale-deflection rating of 500 μA, which makes it one tenth as sensitive as the Simpson. It has no maker’s name and is on long-term loan to the Greenslade Collection from Appalachian State University.
- Thomas B. Greenslade, Jr., “Galvanometers”, Phys. Teach., (1997) 35, 423-426. ↩
- Thomas B. Greenslade, Jr, “Edward Weston and the “Modern” Galvanometer Movement”, Phys. Teach., (2008) 46, 162-164. ↩
- Thomas B. Greenslade, Jr., “The St. Louis Motor”, Phys. Teach., (2011) 49, 424-425. ↩
- Thomas B. Greenslade, Jr. “Apparatus Manufacturers in Chicago, ca. 1900”, Rittenhouse, (1999) 13, 16-19. ↩