LSSl.l 



oil t/ic P/iciioiiiciKi of tlir Elfclric Dischai'ijc. 



481 



of the galvjinoiiK'tcr. Tliis sliows tliat tlio discharj^c in tlio vacuum tube, 

 altliougli apparently quite steady, is a pulsating one ; as tlie swing is 

 to the right we know tliat the currc^nt is a direct or break contact ono, 

 thus indicating that the diseliarge through the tube increases compara- 



Fjg. 18. 



J^JOSULUJLU^^ 



lively slowly, then drops more suddenly. If the rise and fall were 

 in equal times, there would be no deflection of the galvanometer. 



If the terminals of a telephone are placed in the circuit between 

 the battery and a vacuum tube, the pulsations are sometimes suffi- 

 ciently slow to produce audible sounds when the telephone is placed 

 to the ear. But the telephone is not adapted to render evident inter- 

 mittences of a very high order. 



There is a remarkable phenomenon which occurs when a charge 

 is sent through a closed vessel containing air or gas, within certain 

 limits of pressure, which I will endeavour to show you. As soon as 

 the connection is made between tlie battery and the terminals a 

 sudden expansion of gas takes place, as you will see (Fig. 19) by the 

 depression of the mercury in the gauge connected with the bell jar, 

 and as soon as the connection with the battery is broken the gas 

 returns suddenly almost exactly to its original volume, showing only 

 a small increase due to a slight elevation of temperature ; the 

 mercury in the gauge rises, therefore, nearly to its original position. 

 The effect is similar to that which would be produced if an empty 



