128 PHYSIOLOGICAL PHYSICS. [Chap. xn. 



A further development of the experiment may be 

 made by using two galvanometers connected each with 

 zinc troughs. Then arrange a long nerve so that it is 

 in contact with the two troughs of one galvanometer 

 at one end, and that its other end is laid in the usual 

 fashion on the troughs connected with the second 

 galvanometer. The galvanometers each indicate a 

 current. The middle of the nerve is laid on the 

 exciting electrodes, and a constant current passed 

 through it, when at once the two needles indicate a 

 current, the deflection of the one being increased, that of 

 the other diminished, the one end of the wire being in 

 the positive, the other in the negative, phase. This 

 shows that in electrotonus a new electromotive force 

 is produced which adds itself to the natural nerve 

 current at the end of the nerve where the direction of 

 both coincide, and subtracts from the natural nerve 

 current at the end where the direction of both 

 differs. 



The employment of the galvanometer for measuring 

 time and resistances is described in chapters xii. 

 and xiii. 



CHAPTER XII. 



THE GALVANOMETER AS A MEASURER OF TIME. 



IT has been seen (page 102) that by means of the 

 accessory magnet the period of oscillation of the 

 needle of the galvanometer can be made very large, 

 and then the deflection of the needle, under the 

 influence of a current, occurs very slowly. If a 

 current lasting a very short time in proportion to the 

 period of the needles be sent through the galvano- 

 meter, the current will have ceased before the needle 



