The Earliest Electromagnetic Instruments Part 2

Schweigger, therefore, described the basic multiplier idea clearly enough in his first paper, but offered no sketch of the simplest construction until the third paper. In the second paper, meanwhile, he had ill.u.s.trated two peculiar designs involving the principle in less elementary ways.

His indifference to whether the wire loops lie _in_ the magnetic meridian (fig. 3) or perpendicular to it (fig. 5) or "at any other arbitrary angle to it," reveals a poor appreciation of the measuring-instrument potentialities. His conception seems to be primarily that of a detector.

Poggendorf's invention, as first reported by Erman and presented to a wider audience by Gilbert[26] was described as consisting of typically 40 to 50 turns of 1/10-line diameter, silk-covered copper wire tied tightly together, with the whole pressed laterally to form an elliptical opening in which a pivoted compa.s.s needle could move freely while maintaining clearance of about 2 lines from the wire at all points.[27]

"This magnetic condenser can be a great boon to electro-chemistry," said Erman, for "it avoids all the difficulties of electric condensers." He noted that, using the condenser, Poggendorf had already established the electric series for a great number of bodies, discovered various anomalies about conductivities, and found a way of detecting dissymmetry of the poles of a compa.s.s needle. On the other hand, even with the condenser, no magnetic effects have so far been obtainable from a strong tourmaline, or from a 12,000-pair, Zamboni dry cell.

Poggendorf's own account of his work finally appeared as a very long article in the journal known as "Oken's Isis."[28] The editorial controversies mentioned earlier may have occasioned this use of a periodical of such minor status in the fields of physics and chemistry.

The source of Poggendorf's vision of the multiplier principle was a little different from Schweigger's inspiration. Aiming at some detailed a.n.a.lysis of Oersted's observation, Poggendorf ran the connecting wire of his cell-circuit along a vertical line to just above or below the pivot-point of the compa.s.s needle, then, after a right-angle bend, horizontally above or below one of the poles of the needle. As he studied the deflections produced for all four possible positions of such a wire, with both cell polarities, he came to realize that if a rectangular wire loop in a vertical plane enclosed a compa.s.s needle, all parts of the horizontal sides of the loop would produce additive deflections. By a separate experiment, he showed that the vertical sides of the loop would also increase the deflections. He saw at the same time that the effect of additional turns would be c.u.mulative.

The multiple surrounding of the needle by a silk-covered wire, in a plane perpendicular to the long axis of the needle, affords the physicist a very simple and sensitive means of detecting the slightest trace of galvanism, or of magnetism produced by it, so that I have given the name of magnetic condenser to this construction, though I attach no special value to this name ...

In a.n.a.lyzing the astonis.h.i.+ngly increased power which the condenser gives to the magnetic effect of a circuit, the first question that arises is how the effect varies with the number of turns, whether it increases indefinitely or reaches a maximum beyond which additional turns have no effect. The answer to this first question is linked to the solution of another, viz, whether the degrees deflection are a direct expression of the measure of the magnetic force or not.

To instruct myself on this point I made use of three separate circuits, each containing an 8-turn condenser, and put these as close together as possible in the magnetic meridian ... with the needle between the windings. Each single circuit ... gave a deflection of 45 ... When two were connected the deflection was 60, and when finally all three were put in magnetic operation, the deflection grew to only 70. It appears clearly from this that the angle of deflection is not in a simple ratio with the magnetic force acting on the needle....

Neither Poggendorf nor Schweigger seems to have ruled out, on logical grounds alone, the possibility of deflections greater than 90, with the loop-plane in the magnetic meridian, though Poggendorf does add a vague note that if the needle deflected too far it would encounter forces of the opposing sign.

Poggendorf experimented with the size of the circuit wires, finding that larger wires led to greater deflections. He noted that the size of the cell plates and the nature of the cell's moist conductors would certainly have a great effect, but that to investigate these in detail would take undue time, and he therefore proposed to keep this part of the apparatus constant, using one pair of zinc and copper plates 3.6 inches in diameter, separated by cloth soaked in ammonium-chloride solution.

Poggendorf's princ.i.p.al quant.i.tative study of his magnetic condenser used 13 identical coils, each with 100 turns. In order that the turns should all be at approximately the same distance from the needle, the coils were wound of the finest bra.s.s wire that could be silk-insulated, the wire diameter being 0.02 lines. On adding coils one at a time across the cell (i.e., connecting them in parallel), the deflections were as follows:

Turns 100 200 300 400 500 600 700 Deflection in degrees 45 50 55 59-60 62 63 64

Turns 800 900 1000 1100 1200 1300 Deflection in degrees 65 65-1/2 66 66 66 66

Adding some coils with fewer turns, and connecting various combinations "as a _continuum_" (i.e., in series), the deflections using the same cell were:

Turns 1 5 10 25 50 75 100 200 Deflection in degrees 10 22 27 30 35-40 40 40 40

Turns 300 400 500 600 700 800 900 1000 Deflection in degrees 40 40 41 40 40 40 40 40

Making a few coils from wire with 1/8-line diameter, the deflections, again using the same cell were:

Turns 5 25 50 100 Over 100 Deflection in degrees 20-22 40-45 45 65 65

Since the needle used in these experiments was almost as long as the inside clearance of the coils, no simple tangent law can be applied, and it is not possible to discover an equivalent circuit in modern terms.

However, the constancy of the deflections for large numbers of turns in each case indicates that the cell voltage and resistance were fairly constant, and a rough estimate suggests that the cell resistance was comparable to the resistance of one of the 100-turn coils of fine wire.

Such a value means that cell resistance limited the maximum deflections for the parallel-connected multipliers, while coil resistance fixed the limit in the series case.

For all of these reasons, it was impossible that any useful functional law could be obtained from the data.

Poggendorf concluded only that "the amplifying power of the condenser does not increase without limit, but has a maximum value dependent on the conditions of plate area and wire size." He added two other significant comments derived from various observations, that the basic Oersted phenomenon is independent of the earth's magnetism, and that the phenomenon is localized, i.e., is not affected by distant parts of the circuit.

Only a small fraction of Poggendorf's paper is devoted to elucidating the properties of the condenser. A similar amount is concerned with refuting various proposals, such as those of Berzelius and Erman, about distributions of magnetic polarity in a conducting wire to account for Oersted's results. More than half of the paper describes results obtained by using the condenser to compare conductivities and cell polarities under conditions where no effect had previously been detectable. Notable is the observation of needle deflections in circuits whose connecting wires are interrupted by pieces of graphite, manganese dioxide, various sulphur compounds, etc., materials which had previously been considered as insulators in galvanic circuits. Poggendorf gives these the name of "semi-conductor" (_halb-Leiter_).

[Ill.u.s.tration: Figure 6.--ELECTROMAGNETIC INSTRUMENTS OF JAMES c.u.mMING, used at Cambridge in 1821. One is a single-wire "galvanometer,"

following Ampere's definition. c.u.mming called the multiple-turn construction "galvanoscopes." He showed how to increase their sensitivity by partial cancellation of the earth's magnetism at the location of the compa.s.s needle. (From _Transactions of the Cambridge Philosophical Society_, vol. 1, 1821.)]

c.u.mming's first mention of the multiplier phenomenon, in his paper of April 2, 1821,[22] is quite casual, and describes only a one-turn construction. He speaks first of single-turn ring of thick, bra.s.s wire, and after noting that the sides of a circuit produce additive effects on a needle, he comments that a flattened rectangular loop produces nearly quadruple the effect of a single wire. The paper is primarily a review of Oersted's work, with references to electromagnetic observations before Oersted, and accounts of various related but nonmultiplier experiments that c.u.mming has made. His second paper, of May 21st, contains a fine plate (fig. 6) ill.u.s.trating arrangements used in investigating the subject of the paper's t.i.tle "The Application of Magnetism as a Measure of Electricity." (Neither Poggendorf nor any of his commentators ever ill.u.s.trated his "condenser.")

Although this plate is never referred to in the paper itself, a nearby "Description" gives a few comments. The two wire patterns shown are noted as simply "forms of spiral for increasing the electromagnetic intensity." The mounted wire loop, with enclosed compa.s.s needle and terminal mercury cups, is clearly identical in principle with the devices of Schweigger and Poggendorf, and is called a "galvanoscope."

The largest structure ill.u.s.trated does not involve the multiplying effect. It is called a "galvanometer," consistent with Ampere's definition of that word. To use it, two leads of a voltaic circuit are inserted into the mercury cups AC and BD, and the board EFGH carrying the cups is moved vertically until some "standard" deflection is obtained on the compa.s.s needle below. The relative "strength" of the circuit is then given by the calibrated position of the sliding section.

Uncertainties are undoubtedly introduced by the arbitrary positions of the connecting wires from the test circuit to the mercury cups, but c.u.mming drew some interesting conclusions from various measurements he made.

Observing needle deflections for various positions of the wire A-B, with a "constant" voltaic circuit, he found that "the tangent of the deviation varies inversely as the distance of the connecting wire from the magnetic needle." Here is a combination of the deflection law for a needle in a transverse horizontal field and the magnetic-force law for a long, straight wire. The latter had been determined experimentally by Biot and Savart, in November 1820, by timing the oscillations of a suspended magnet.[29]

c.u.mming considers his straight-wire calibrated "galvanometer" to be a device for "measuring" galvanic electricity; on the other hand, his multiple-loop "galvanoscopes" are for "discovering" galvanic electricity. With the multiplier instrument, he found galvanic effects (i.e., needle deflections) using copper and zinc electrodes with several acids not previously known to create galvanic action. A pota.s.sium-mercury amalgam electrode created a powerful cell with zinc as the positive electrode, establis.h.i.+ng both the metallic nature of pota.s.sium and the fact that it is the most negative of all metals.

In a third paper, presented April 28, 1823,[30] c.u.mming reports use of the galvanoscope in experiments on the thermoelectric phenomena recently discovered by Seebeck. His note that "for the more minute effects a compa.s.s was employed in the galvanoscope, having its terrestrial magnetism neutralized ..." seems to be the earliest mention of this version of the astatic principle, a technique whose dramatic effects were especially valuable in low-resistance thermoelectric circuits, where the extra resistance of additional multiplier turns largely offsets their magnetic contribution. In detail, "the needle is neutralized by placing a powerful magnet North and South on a line with its center; and another, which is much weaker, East and West at some distance above it: by means of the first the needle is placed nearly at right angles to the meridian, and the adjustment is completed by the second."

On varying the length of the connecting wire of the circuit, c.u.mming found the deflections of the multiplier needle to be in a nearly reciprocal relation. He speaks of the "conducting power of the wire,"

and seems not far from visualizing Ohm's law, of which no published form appeared until 1826. Ohm's own experiments were made with very similar apparatus.

[Ill.u.s.tration: Figure 7.--"SCHWEIGGER MULTIPLIER" used by Oersted in 1823. A thin magnetic needle is held in a light, paper sling at F, suspended by a fine, vertical fiber. (From _Annales de Chimie et de Physique_.)]

Conclusions

An effort has been made to show that electrical experimenters prior to Oersted's discovery in 1820 were in desperate need of some electrical instrument for galvanic or voltaic circuits that would combine sensitivity, simplicity, reliability, and quick response. The nearly simultaneous creation by Schweigger, Poggendorf and c.u.mming of an arrangement consisting of a coil of wire and a compa.s.s needle provided the first primitive version of a device to fill that need.

[Ill.u.s.tration: Figure 8.--COMPLETELY USELESS ARRANGEMENT of vertical coil and horizontal, unmagnetized needle, presented in the _Edinburgh Philosophical Journal_ of 1821 as "Poggendorf's Galvano-Magnetic Condenser." Almost every aspect of Poggendorf's instrument has been incorrectly represented.]

It appears that Schweigger is clearly ent.i.tled to credit for absolute priority in the discovery, but the original sources suggest that both his understanding of the device and the subsequent researches he performed with it were markedly inferior to those of the other independent discoverers. In using the generic label, "Schweigger's Multiplier," there have been historical examples of attributing to Schweigger considerably more sophistication than is justified. Figure 7 shows an instrument designed by Oersted in 1823,[20] which he says "differs in only minor particulars from that of M. Schweigger." On comparing figure 7 with figures 3, 4, or 5, the remark seems overly generous.

The history of the multiplier instruments has had its fair share of erroneous reports and misleading clues. A fine example is the ill.u.s.tration of figure 8, taken from what is often quoted as the first report in English on Poggendorf's "Galvano-Magnetic Condenser."[31] The sketch is the editor's interpretation of a verbal description given him by a visiting Danish chemist who, in turn, had received the information in a letter from Oersted. It incorporates, faithful to the description, a "spiral wire ... established vertically," with a needle "in the axis of the spiral," yet by misunderstanding of the axial relations and of the ratio of length to diameter for the coil, a completely meaningless arrangement has resulted. The confusion is compounded by the specifying of an _unmagnetized_ needle.

Schweigger and Poggendorf, through their editorial positions, were among the best known of all European scientists for several decades. On one basis or another their reputations are firmly established. Comparison of the accounts of the early "multipliers," however, suggests that the Reverend James c.u.mming, professor of chemistry at the University of Cambridge, was a very perceptive philosopher. This was well understood by G. T. Bettany who wrote in the _Dictionary of National Biography_ that c.u.mming's early papers "though extremely unpretentious," were "landmarks in electromagnetism and thermoelectricity," and concluded that: "Had he been more ambitious and of less uncertain health, his clearness and grasp and his great apt.i.tude for research might have carried him into the front rank of discoverers."

ACKNOWLEDGMENTS

I wish to thank Dr. Robert P. Multhauf, chairman of the Department of Science and Technology in the Smithsonian Inst.i.tution's Museum of History and Technology, for encouragement in the writing of this paper and for the provision of opportunity to consult the appropriate sources.

To Dr. W. James King of the American Inst.i.tute of Physics, I am grateful for many provocative discussions on this and related topics.

FOOTNOTES:

[1] A. VOLTA, "On the Electricity Excited by the Mere Contact of Conducting Substances of Different Kinds," _Philosophical Transactions of the Royal Society of London_ (1800), vol. 90, pp. 403-431.