THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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. 39 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 



Fig. 40. 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 (n) north pole, and (s) 

 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. 41. 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. 



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. 



The movable system of this galvanometer possesses considerable 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. 



