240 



Professor J. J, Thomson 



[April 13, 



By taking a long discharge tube, say, one 50 feet long, and observing 

 the luminous discharge through a rotating mirror, we can trace the 

 course of the luminosity due to a single discharge, say, one due to 

 once breaking the primary circuit of an induction coil ; if we do so 

 we find that the luminosity follows the direction of the positive 

 current through the tube. That is, the luminosity begins at the 

 positive electrode, it then rushes down the tube with enormous 



velocity, but when it 

 & 1- gets to the negative 



^____ electrode, it receives a 



c-(^ ~j ~3"3 check ; it does not dis- 



appear at once in that 



electrode like a rabbit 

 going down a hole, but 

 lingers around the elec- 

 trode some time before 

 entering it. In conse- 

 quence of this delay in 

 the positive discharge 

 in getting out of the 

 gas, there is an accumu- 

 lation of positive elec- 

 tricity in the neighbour- 

 hood of the negative 

 electrode until the po- 

 tential fall at this elec- 

 trode increases to about 

 200 or 300 volts. 



The positive electri- 



Icity which accompanies 

 the discharge thus finds 

 considerable difficulty 

 in getting from the gas 

 to the metal, though, as 

 I hope to show you later 

 on, as long as it keeps 

 in the gas, it meets^with 

 what we may, in con- 

 sideration of the views 

 sometimes enunciated on this subject, call a ridiculously small amount 

 of resistance, its real difficulty is to get out of the gas. 



Though this effect has long been known, it is so important that I 

 will venture to show one or two experiments which illustrate it. 

 The arrangement of the first experiment is shown on the screen, the 

 apparatus consists of a main discharge tube, across which is fastened 

 a diaphragm made of excessively thin platinum leaf; there is a side 

 passage from the tube, leading from one side of the diaphragm to the 

 other, this is connected to a barometer tube, and by raising the 



