94 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 



Fig. 39. Diagram of struc- 

 ture of the d'Arsonval galvanom- 

 eter, c is the coil of fine wire 

 through which the current is 

 passed. It is swung by a fine 

 thread of phosphor-bronze so as 

 to lie between and close to the 

 poles (ri) north pole, and () 

 south pole of the magnet. Just 

 above the magnet the thread car- 

 ries a mica or aluminum vane to 

 which is attached a small mirror. 

 The scale of the instrument is re- 

 flected in this mirror and is 

 observed through the telescope 

 shown in Fig. 38. 



Fig. 40. Schema of capillary electrometer 

 arranged to show the demarcation current in 

 muscle (Lombard) : a, The glass tube containing 

 mercury and drawn to a fine capillary below; c, 

 the receptacle containing mercury by raising 

 which the mercury can be driven into the capil- 

 lary of a; /, a vessel with glass sides containing 

 mercury below, and above dilute sulphuric acid 

 into which the capillary of a dips; E, the micro- 

 cope for observing the mercury thread in the 

 capillary; m, the muscle; g and h, the wires 

 touching the longitudinal and cut surfaces of the 

 muscle. The current flows as indicated by the 

 small arrows; d, the capillary thread of mercury 

 as seen under the microscope. 



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. 



The Capillary Electrometer. The movable system of a galvanometer 

 possesses considerable weight, therefore inertia; so that it will not indicate 

 accurately the presence or extent of very brief electrical currents such as have 

 to be studied in physiology in some cases. For purposes of this kind a simple 



