ELECTRICAL PHENOMENA. 



93 



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 

 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 the complete instrument in Fig. 38. A large horse- 

 shoe 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 





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



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 

 reflected in this mirror. 'As the coil and mirror are twisted by the action 

 of the current passing through the former the reflection of the scale in the 

 mirror is displaced. By means of a cross hair in the telescope the angle of 

 deflection may be read upon the reflected scale. The aluminum vane back of 

 the mirror makes the system dead-beat so that when a deflection is obtained 

 the system comes quickly to rest with few or no oscillations. If the coil of wire 

 contains sufficient turns, enough to give a total resistance of two to three 

 thousand ohms, and the poles of the magnet are brought very close to the 

 coil, the instrument may be given a delicacy sufficient to study accurately 

 the muscle and nerve currents. In such an instrument the effect of the earth's 

 magnetism may be neglected and the galvanometer may be hung upon any 

 support without reference to the magnetic meridian. 



