ELECTRICAL PHENOMENA. 



97 



Means of Demonstrating the Muscle Current. The demarcation 

 current and other electrical conditions to be described require especial appara- 

 tus for their study. To detect the existence of a current physiologists use 

 either a galvanometer or a capillary electrometer. The galvanometers employed 

 are of several types, the Kelvin reflecting galvanometer, the d'Arsonval form, 

 and more recently the " string-galvanometer" of Einthoven. The principle 

 of the galvanometer lies in the fact that a magnetic needle is deflected when 

 an electrical current passes through a wire in its vicinity. If a magnetic 

 needle is swung by a delicate thread so as to move easily, it will come to rest 

 in the magnetic meridian with its north pole pointing north. If now a wire is 

 curved round it, as shown in the accompanying diagram (Fig. 38), and a battery 

 current is sent through this wire, the needle will be deflected to the right if the 

 current 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 and 

 direction of the electrical current in the wire. The extent of deflection of 

 the needle may be used to measure the strength of the current by ascertaining 



Fig. 39. D'Arsonval galvanometer as modified by Rowland. 



the amount of deflection caused by a standard battery. The effect of the 

 current upon the needle increases with the number of turns of wire, so that 

 delicate galvanometers constructed upon this principle are spoken of as high 

 resistance galvanometers, the great length of wire used making, of course, a 

 high resistance. Instead of having the coil through which the current passes 

 kept in a fixed position and the magnet delicately swung or poised, the reverse 

 arrangement may be used that is, the coil may be swung between the poles 

 of a fixed magnet. Under these circumstances, if a current is sent through the 

 coil, this latter will move with reference to the magnet. A galvanometer con- 

 structed on this principle is designated as a d'Arsonval galvanometer, after 

 the physiologist who first employed this arrangement. In the d'Arsonval 

 form the magnet is fixed while the coil of wire through which the current 

 passes is swung by a very delicate thread of quartz, silk fiber, or phosphor- 

 bronze. The principle of the arrangement is shown in the accompanying 

 diagram (Fig. 40) and one form of a complete instrument in Fig. 39. A large 

 horseshoe magnet (n, s) is fixed permanently and between the poles is swung 

 a coil (c) of delicate wire, the two ends of the wire being connected with binding 

 posts in the frame of the instrument. The coil is held in place below by a 

 delicate spiral. In Fig. 40 it will be seen that the delicate thread suspending 



7 



