. A text-book of physiology : for medical students and physicians . useeither a galvanometer or a capillary electrometer. The galvanometers employedare of several types, the Kelvin reflecting galvanometer, the dArsonval form,and more recently the string-galvanometer of Einthoveh. The principleof the galvanometer lies in the fact that a magnetic needle is deflected whenan electrical current passes through a wire in its vicinity. If a magneticneedle is swung by a delicate thread so as to move easily, it will come to restin the magnetic meridian with its north pole pointing north. If now a wire i
. A text-book of physiology : for medical students and physicians . useeither a galvanometer or a capillary electrometer. The galvanometers employedare of several types, the Kelvin reflecting galvanometer, the dArsonval form,and more recently the string-galvanometer of Einthoveh. The principleof the galvanometer lies in the fact that a magnetic needle is deflected whenan electrical current passes through a wire in its vicinity. If a magneticneedle is swung by a delicate thread so as to move easily, it will come to restin the magnetic meridian with its north pole pointing north. If now a wire iscurved round it, as shown in the accompanying diagram (Fig. 38), and a batterycurrent is sent through this wire, the needle will be deflected to the right if thecurrent passes in one direction and to the left if it passes in the opposite direc-tion. The movement of the needle is an indication of the presence anddirection of the electrical current in the wire. The extent of deflection ofthe needle may be used to measure the strength of the current by ascertaining. Fig. 39.—DArsonval galvanometer as modified by Rowland. the amount of deflection caused by a standard battery. The effect of thecurrent upon the needle increases with the number of turns of wire, so thatdelicate galvanometers constructed upon this principle are spoken of as highresistance galvanometers, the great length of wire used making, of course, ahigh resistance. Instead of having the coil through which the current passeskept in a fixed position and the magnet delicately swung or poised, the reversearrangement may be used—that is, the coil may be swung between the polesof a fixed magnet. Under these circumstances, if a current is sent through thecoil, this latter will move with reference to the magnet. A galvanometer con-structed on this principle is designated as a dArsonval galvanometer, afterthe physiologist who first employed this arrangement. In the d Arsonvalform the magnet is fixed while the coil of w
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