544 



Mr. A. Schuster. 



dY Ia 2 



-— 4an? 9 

 ax I 



where dYjdx stands for the fall of potential at the kathode. In my ex- 

 periments this quantity has been smaller than 10 12 C.Gr.S., so that the 

 maximum total charge per centimeter cross section would amount to 

 about three electrostatic units. The effect of this electrification 

 would be completely hidden by the charges on the outside of the 

 glass tube, which cannot be avoided when we are dealing with a 

 thousand volts. In fact, the leakage over the glass will act as an 

 electrical screen. 



It also appears that electrified bodies outside cannot produce any 

 effect on the inside of the tube, because it can be shown from my 

 previous work that the surface conditions in the interior of a gas 

 through which a discharge passes render a normal force at the 

 surface impossible. Hence no lines of force can pass from the out- 

 side to the inside. If an electrified body is approached, a momentary 

 redistribution of the surface charge inside the vessel will take place ; 

 but after the readjustment no further effect can be observed. 



It is generally stated that a greater electromotive force is required 

 to start a discharge than to maintain it after it is once started. This 

 statement requires qualification. The fall of potential in the positive 

 part of the tube is of course much smaller during discharge than it was 

 just before, but it does not follow that the rate of fall at the negative 

 electrode is less. In one experiment the rate of fall at the kathode 

 at a pressure of 6 mm. was 9000 volts per centimeter, while the 

 whole of my battery only had a difference of potential of less than 

 2000 volts. According to De la Rue and Miiller, it requires a fall of 

 about 1800 volts to start a discharge between two parallel plates at 

 a pressure of 10 mm. Their measurements do not extend to smaller 

 pressures, but a discharge would be started at the pressure of about 

 \ mm. by a less electromotive force than at a pressure of 10 mm. It 

 follows that while the discharge is passing, in most if not in all my 

 experiments, the fall of potential was much greater than that required 

 to start it, and if we adopt the view that the breaking down of the 

 insulation is in part due to a decomposition of the molecules, it 

 follows that the molecules must continue to be decomposed during 

 the discharge. This we shall find to be a result of importance. 



The charge on platinum electrodes in liquids is much greater than 

 the charge on the surface of the kathode in a gas ; with the usual data 

 I make it about 10,000 times greater. This would explain the 

 absence of observable polarisation in the case of the gas discharge, 

 but it seems surprising that any electricity should pass at all under 

 the circumstances, between the gas and the electrode. My calcula- 

 tions, however, depend on the extension of an experimental formula 



