540 



Dr. John HopMnson 



[April 26, 



of one cell giving about two volts. You observe that the current 

 induced in the secondary coil takes a somewhat long time for its 

 production and for its subsequent diminution — about 15 seconds. I 

 will now connect it to a battery of high E.M.F. or rather to the 

 mains which supply current to the building, and we will repeat the 

 experiment with the same current in the primary coil. Observe the 

 difference. The current increases more rapidly and dies away very 

 much more quickly, taking in all about three seconds instead of 15 

 seconds, as before. I should perhaps here state that the sensibility of 

 the galvanometer has been altered between the two experiments, so as 

 to give a deflection of convenient size to be observed upon the screen. 



Fig. 1 



In the first case it is very much more sensitive than in the second. 

 Exactly the same experiment can be shown to you in another form. 

 These galvanometer deflections can be exhibited in the form of curves, 

 in which the abscissae represent times and the ordinates represent the 

 deflections. The curve which I am showing you (Fig. 1) has actually 

 been taken as a photograph, the plate being moved across the field 

 with uniform velocity. On the same plate you see the curves which 

 have been obtained with a larger E.M.F. and with a smaller one. 

 The times at which the reversal occurred in each case are marked by 

 the sudden change at the beginning of the curve. You see exactly 

 the same thing as you saw upon the screen : the big battery makes 

 the changes occur rapidly, the small battery allows them to occur 



