[The first half of this paper describes successful observations of the
reaction-force of a magnetic needle on the connecting wire of a voltaic
circuit, achieved by pivoting the connecting wire in the form of brass
needles above and below the compass needle. Though the multiplier
configuration of needle and wire is in fact present here, Schweigger
does not mention it, evidently regarding this as a separate project. He
continues.]
In my lecture of September 16th, I showed that Oersted's results
depend, not on the voltaic cell, but only on the connecting circuit.
The principle I have used for amplification of the effects, for the
construction of an electromagnetic battery as it were, was the
winding of wire around the compass, and I now present to the Society
a bow-pattern of multiple-wound, wax-insulated wire, Figure 3.
[There were no illustrations with Schweigger's first paper.] While
a single wire, using the weak electric circuit here, deflects the
magnetic needle only 30° or 40°, if the compass is placed in one of
the openings of this pattern, the needle is deflected 90° to the
east, or in the other opening 90° to the west, using the same weak
electric circuit....
The "bow-pattern" device has novelty interest only, adding nothing to
the elucidation of the multiplier phenomenon. The same is true of
Schweigger's next proposal, shown in figure 4. "... I will now add
another apparatus, which is just an extension of the previous one,
whereby the needle can take up any angle from 0° to 180°." A short
length of circular glass tubing, of inside diameter large enough to
contain a compass needle, stands with its axis vertical and has single
or multiple loops of wire wound on it in vertical diametral planes. In
the illustration, successive plane coils are inclined at 30° to one
another. "... the electric current flows through the whole wire, and the
needle moves under all of these currents, and coming always into another
loop can take any desired angle."
With much further theorizing about "the correlation of magnetism with
the cohesion of bodies," Schweigger states again his evaluation of his
discovery: "Oersted succeeded in electromagnetic research by using a
spark-producing cell, which could make a wire glow. My amplifying
electromagnetic device needs only a weak circuit of copper, zinc, and
ammonium chloride solution."[24]
[Illustration: Figure 4.--SCHWEIGGER MADE THIS peculiar construction of
wire coils, wound endwise on a short vertical section of glass tubing
with a compass needle inside, merely to startle his Halle audience with
the fact that the compass needle could rest in any of several stable
positions. (From _Journal für Chemie und Physik_.)]
[Illustration: Figure 5.--SCHWEIGGER'S SUGGESTION of one possible design
for an amplifying electromagnetic indicator. The components are wooden
rods and insulated wire. Position b referred to in the text is at the
bottom of the diagram between the letters a and c. (From _Journal für
Chemie und Physik_.)]
"FURTHER WORDS ABOUT THE NEW MAGNETIC PHENOMENA"
[This was presumably written between November 4, 1820, and the January
1, 1821, publication date of his _Journal_.]
These wonderful new electrical effects[25] are most easily rendered
perceptible with the help of the previously described wire loops. To
focus attention on just one of the windings of Figure 3, we sketch a
new drawing, Figure 5.... Since it is of major importance that these
loops be made of silk-covered wire lying evenly on one another, it
is convenient to wind the loops on two small slotted sticks of wood,
although it is also possible to hold the wires together with wax or
shellac, or to tie them together in an orderly manner with silk
thread....
In Figure 5, Aa and Cc represent little slotted rods of wood on
which the silk-covered wire is wound. Only three windings are shown
in the figure, but I generally adopt three times that many. Now t is
connected with the copper and d with the zinc, and the compass B set
between the rods Aa and Cc with the coil perpendicular to the
magnetic meridian and the terminals d, t at the east.
The instant Z and K are dipped in the ammonium chloride solution,
the needle turns around and stays with the north pole point
south....
If now the compass is taken out of the coil and put in position b,
all effects are reversed, and are considerably weaker, for obvious
reasons....
It is of the same significance whether we bring the compass from B
to b in Figure 5, or from mesh 1 to mesh 2 in Figure 3, only that in
the latter case, because the compass is enclosed by the two sides, a
stronger effect results....
If now the coil is rotated ... so that the face previously north now
faces south, then on connecting the electric circuit there is
absolutely no trace of effect on the needle, assuming that the
terminal wires are not reversed....
It seems unnecessary to note that our magnetic coil can be placed in
the direction of the magnetic meridian or at any arbitrary angle
with it....
Following several pages of further talk about the relation of "cohesion
to magnetism" and about "unipolar and bipolar conductors," the only
additional item of interest is the observation that discharges of a
Leyden jar (_Kleistichen Flasche_) strong enough to burn strips of leaf
gold and to magnetize an iron rod in a coil, produced no compass-needle
deflections, even with the help of the "amplifying apparatus."
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 illustrated 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 compass 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 compass 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
analysis 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 compass 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 compass 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 cumulative.
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 analyzing the astonishingly 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 principal quantitative 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 brass 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_).
[Illustration: Figure 6.--ELECTROMAGNETIC INSTRUMENTS OF JAMES CUMMING,
used at Cambridge in 1821. One is a single-wire "galvanometer,"
following Ampère's definition. Cumming 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 compass needle. (From _Transactions of the Cambridge
Philosophical Society_, vol. 1, 1821.)]
Cumming'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, brass 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 Cumming has made. His second paper, of May 21st,
contains a fine plate (fig. 6) illustrating arrangements used in
investigating the subject of the paper's title "The Application of
Magnetism as a Measure of Electricity." (Neither Poggendorf nor any of
his commentators ever illustrated 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 compass 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 illustrated does not involve the multiplying
effect. It is called a "galvanometer," consistent with Ampère'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 compass 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
Cumming 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]
Cumming 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
potassium-mercury amalgam electrode created a powerful cell with zinc as
the positive electrode, establishing both the metallic nature of
potassium and the fact that it is the most negative of all metals.
In a third paper, presented April 28, 1823,[30] Cumming reports use of
the galvanoscope in experiments on the thermoelectric phenomena recently
discovered by Seebeck. His note that "for the more minute effects a
compass 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, Cumming
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.
[Illustration: 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_.)]