60 



EXPERIMENTAL PII YSIOLOG Y 



dips into the mercury, the circuit is closed, and consequently the electromagnet 

 attracts the 'iron reed, the point of which is thus lifted from the mercury, and 

 the circuit is broken. The reed then falls, the current is again made, and the 

 cycle repeated indefinitely. Note that the general principle of the whole 

 arrangement is precisely the same as that of the Neef's hammer attached to 

 the inductorium. The only difference that we have is, that the length of the 

 reed, and consequently the rate at which it vibrates, is easily adjustable, 

 whereas in the Neef s hammer the rate of vibration is permanently fixed. 

 In some ways it is rather more convenient to fix the reed so that it vibrates 

 horizontally instead of vertically, as in fig. 61. The platinum point is then 

 bent downwards and the mercury cup overfilled, and so adjusted that the 

 platinum point makes contact by touching the side of the mercury. By 

 using it in this position there are fewer adjustments to be made when the 



IZ3EZ 1 '. . '. IS :iSE L.' J L ; l Hi 15 



Fig. 60. — Mode of Fitting dp a Vibrating Reed. 



spring has to be changed to a fresh position, and it will be found that the spring 

 vibrates rather more regularly. The wire connections are the same as in the 

 previous case. 



In using the reed each time the platinum dips into the mercury, a make 

 shock is produced in the secondary ; and each time it is lifted from the 

 mercury a break shock is produced, so that there are two stimuli for each 

 complete vibration, which are, moreover, of unequal value. This is a great 

 disadvantage in the employment of the vibrating reed which can, to a certain 

 extent, be overcome by so regulating the position of the secondary relatively 

 to the primary that only the break shocks are effective in producing a twitch. 

 Even then, however, we must not consider the make shock as producing no 

 ■effect, for it is found that a shock which in itself is insufficient to cause 

 a contraction can produce an alteration in the nerve or muscle, which is 



