94 



THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 

 instrument known as the capillary electrometer is employed. The principle 

 of the construction of this instrument is illustrated in Fig. 40. A glass tube, 

 a, is drawn out at one end into a very fine capillary the end of which dips 



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 (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. 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. 



into some diluted sulphuric acid contained in the vessel (/) . At the bottom 

 of this vessel is a layer of mercury connecting with a wire, g, fused into the 

 glass vessel. The tube a is partially filled with redistilled mercury, which 

 penetrates for a short distance into the capillary. By means of pressure ap- 

 plied from above c, the mercury can be forced through the capillary. Then 

 by diminishing the pressure the mercury can be brought back into the capil- 

 lary a certain distance, drawing after it some of the dilute sulphuric acid. 

 The mercury in tube a is connected with the other pole of the battery by a 



