LATENT PERIOD 



27 



the curve of muscle contraction. Set the drum revolving. 

 Depress for a moment the steel band to the rod (r) 

 beneath it, and bring the writing point to write on the 

 drum for one revolution and then remove it. With the 

 band given you a double vibration (crest to crest of the 

 wave) takes ^ sec. Measure the duration of the latent 

 period of the contraction and of the relaxation. 



b. Connect in one circuit two cells, a key (in-circuit), a tuning-fork 

 vibrating 100 times a second, and the bobbin of a time-marker, as in 

 Fig. 12. 



The current flows from the battery by the wire /, through the 

 tuning-fork, down the pin connected with the lower prong to the 

 mercury cup Hg and so to the binding screw e. Here the current 

 divides into two circuits, one passes through the coil d lying between 

 the prongs, and the other passes by the wires g and b through the 

 coils of a time-marker, and both circuits are brought to the binding 

 screw a, from which the current passes to the battery. 



Fig. 12. 



The two ends of the wire wound round the coil (c) are connected 

 with the binding screws to which the current is conducted. When 

 the circuit is made the current passes round the coil, the soft core of 

 the coil becomes magnetised and the hammer (h) and lever (I) are 

 drawn down; when the circuit is broken the hammer and lever are 

 drawn back by a spring. 



See that the surface of the mercury is quite clean, put on it a 

 few drops of 95 % alcohol, adjust the pin so that it just touches 

 the mercury, flick the tuning-fork, it will continue to vibrate, 



