164 PHYSICS. 
they must exert a deflecting influence 100 times greater than that of a 
single turn. For this purpose a copper wire, fifty or sixty feet long, and 
covered with silk, is wound around a rectangular frame of wood or metal, 
so as to leave the two extremities free; within this frame a magnetic 
needle is to be suspended from a fibre of silk. The entire apparatus, 
covered by a glass receiver, is termed a multiplier, and serves to render 
sensible the feeblest galvanic current, or the least trace of galvanism. 
Nobili, however, rendered the multiplier much more sensitive by employing 
a system of two needles (fig. 70) instead of one: these are combined on a 
straw or thin wire with their similar poles in opposite directions. The 
terrestrial polarity of the needles being thus destroyed, the astatic needle is 
free to obey the deflecting force of the very feeblest trace of galvanism. 
One needle hangs within and the other without the turns of the wire, both 
being thus deflected in the same direction. The upper needle traverses a 
circle graduated to 360°, pointing to 0° when no current passes through the 
coil; the more powerful the current the greater the deflection from this 
position. Pl. 22, fig. 48, represents the whole of an astatic multiplier, and 
fig. 49 the frame with its windings seen from above; n and p are the 
extremities of the windings to be connected with the poles of the 
battery. 
The tangent and the sine compass likewise depend upon the deflecting 
force exercised by the galvanic current on the magnetic needle. They can 
only be used with the more powerful currents, but nevertheless have this 
advantage over the multipliers, that in them the angle of deviation is in 
very simple proportion to the strength of the current. Thus in the tangent 
compass the strength of the current is proportional to the tangent, and in 
the sine compass to the sine of the angle of deviation. Pl. 20, fig. 71, 
represents a tangent compass according to the construction of Weber. 
Here the current is carried around the magnetic needle through a broad 
circular copper strip whose plane must lie in that of the meridian. The 
needle, which need not be astatic, is in the centre of the copper hoop, and 
is very small in proportion to it. The current is carried to the hoop 
through a copper rod, and is brought back through a hollow copper cylinder, 
inclosing the rod without being in conductive contact with it 
(see figs. 72-74): a and b (fig.'71) are the mercury cups in which the 
electrodes are dipped. The sine compass is shown in fig. 75. In this 
instrument the needle is placed in the centre of a horizontal graduated 
circle turning about a vertical axis, and about which the multiplying wire 
(in one or more turns) is wound. The instrument is set up so that the 
plane of the multiplier les in the magnetic meridian. 
Difference between compound and simple batteries.—The actual quantity 
or amount of current electricity is no greater in a compound battery than 
in one of its simple components, provided that the closing of the circuit is 
produced throughout by good conductors; it depends, not upon the number. 
but upon the size of the plates. On the other hand, the tension or intensity 
of the electricity increases with the number of pairs; therefore, in those 
cases where a bad conductor is interpolated in the circuit, as the human 
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