254 



PHYSIOLOGY 



key k is then closed, and by means of the reverser the current is sent through 

 the galvanometer in the opposite direction to the demarcation current, and the 

 rider c shifted until the two currents exactly balance one another, and the 

 needle of the galvanometer returns to zero of the scale. This adjustment is 

 first made, using only -jTjVo ^ ^ e total current, and then by means of the 

 shunt, xcro> Y^> an d finally the whole current is thrown into the galvanometer. 

 If this precaution be not taken, much too large a current may in the first case 

 be sent through the galvanometer, to the detriment of the instrument. 

 If we know the difference of potential between the two ends of the wire, 



the proportion will give us the E.M.F. of the demarcation current. The 



galvanometer needle having by compensation been brought to zero, stimu- 

 lation of the nerve at e by interrupted currents causes the needle to swing 

 at once in the opposite direction to the first variation. This swing is the 

 measure of the negative variation or current of action. 



FIG. 81. 



In order to study the electrical changes accompanying a single muscle 

 twitch, it is necessary to employ some instrument which can react much more 

 rapidly than the ordinary galvanometer. For this purpose we may employ 

 either the capillary electrometer or the string galvanometer of Einthoven. 



The capillary electrometer is an instrument for recording and measuring 

 difference of potential. That is to say, if connected with two points, it measures 

 the force which would make a current flow between these two pointsjif they 

 were connected by a wire. Its structure is very simple. It consists of a 

 glass tube drawn out to a fine capillary point. This tube with the capillary is 

 filled with mercury. The point dips into a wide tube containing dilute sul- 

 phuric acid, at the bottom of which is a little mercury. Two platinum wires 

 melted into the glass and dipping into the mercury serve as terminals. 

 When the instrument is used, the meniscus of the mercury in the capillary 

 at its junction with the acid is observed under the microscope, or a magnified 

 image of it is thrown oi> a screen with the aid of the electric light. If now the 

 qapillary and acid be connected with two points, it will be observed that any 

 difference in the potential of these two points causes a movement of the 



