ELECTRICAL PHENOMENA. 



93 



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, 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 direction. The move- 

 ment 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 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 galvanom- 

 eters, 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 



Fig. 38. D'Arsonval galvanometer as modified by Rowland. 



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. The d'Arsonval form 

 of galvanometer possesses many practical advantages for physiological work, 

 and it may suffice to give the details of this form alone. 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. 39) and one form of a complete instrument in Fig. 38. 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. 39 it will be seen that the delicate thread suspending 

 the coil carries just above the coil a small mirror, m, and a plate of thin mica 

 or aluminum. The mirror is deflected with the coil, and when viewed through 

 the telescope pictured in Fig. 38 the image of the scale above the telescope is 



