chap, xii.] MEASUREMENT OF TIME. 



commutator so as to stimulate at a, and take 

 another reading. There will be a difference between 

 them, indicating a longer period between the moment 

 of stimulation and that of contraction. Obviously 

 this difference is due to the time which the nerve 

 energy liberated at a has taken to travel the distance 

 between a and b. This distance is measured, and thus 

 one has an estimate of the length of time taken by the 

 nervous energy to travel a certain distance, an. 

 estimate, that is, of the rapidity of the nerve current. 

 Thus by means of the frog-interrupter and the gal- 

 vanometer, measurements can be made of the period 

 of latent stimulation, and of the rapidity of the nerve 

 current. 



It is proper to say that the arrangement has been 

 slightly simplified for purposes of explanation. It is 

 not desirable to use the same primary current to 

 establish the current through the galvanometer, and 

 produce at the same time by its closure an induced 

 cm-rent sent to the muscle. One element is used for 

 the galvanometer and interrupter : but the kev of this 



O 1 ' ./ 



circuit is two-sided, and is so arranged that the same 

 instant that it closes the galvanometer-interrupter 

 circuit, it opens the circuit of another element and the 

 primary coil, and so gives an induced current of 

 opening to the muscle. The representation of this 

 would make the diagram seem a little complicated, so 

 the simpler arrangement has been drawn to show the 

 principle of the method. Other methods of measuring 

 the latent period and the rapidity of the nerve cur- 

 rent are considered under the Graphic Method in 

 chapter xvii, 



