PERIOD 01" A PENDULUM. 



30.5 



Traces o\ the action of the electromagnet were then taken 

 011 the tape and examined with a microscope. These form an 

 interesting subject oi study in themselves. They agreed in 

 their general characteristics with the expectations based on the 

 time constant and the period of immersion of the pendulum, 

 e.g., using 4 volts 0.10 ampere 20 mm. semiamplitude, and a cup 

 7 mm. in diameter, the calculated latent period required for 

 current to become large enough to attract the armature of the 

 magnet is about 0.008 s ; the time occupied by the forward 

 movement of the armature was 0.008 s ; it was stationary for 



0.065 s . and the spring caused it to return in o.oir. Whilst the 

 whole time of passage through the cup should he 0.056 s . The 

 amplitude is small in this instance. By increasing it the stationary 

 period can he reduced till it disappears altogether ; hut to do this 

 the amplitude must he made to exceed the value commonly used 

 in the experiments. So there seems little prospect of an explana- 

 tion, on those lines, of the change in period. 



Subsequently I attached a small mirror to the Kater pendu- 

 lum, and observed its vibrations by the usual coincidence appar- 



