181 



Fig. 1. 



the cylindrical ^auze N, after minierons collisions; opposite this 

 cm ganze a receiving plate has been adjusted, 

 also cylindrical. (The cylindrical arrange- 

 ment of iV, and P appeared to be prefer- 

 able, though good results were also ob- 

 tained with an apparatus with two parallel 

 pieces of gauze and a plane receiving plate). 

 When between xV, and Pa. small retarding 

 potential is applied, all the electrons, the 

 velocity of which corresponds to smaller 

 potentials, are held back. A certain part 

 of the faster electrons will likewise be 

 retained by the weak counter-field, but 

 as appears on closer consideration, this part greatly decreases with 

 increasing velocity. The difference between the stream of electrons 

 received on the plate with and without the small counterfield, 

 therefore, gives a measure for the number of electrons having about 

 the velocity zero. In order to be able to measure this difference with 

 great accuracy, an arrangement was chosen which rendered it 

 possible to insert and cut out the field alternately; the part of the 

 potentiometer from which the small counter-potential had been branched 

 off, could be short-circuited by a mercury contact in vacuum. By 

 alternate reading of the deviation with and without counterfield the 

 difference could be accurately read, and an error in consequence 

 of a possible change of the zero-point was out of the question. It is 

 further possible to render oneself independent of a slow change in 

 the emission of electrons of the incandescent wire, by dividing the 

 difference measured by the total deviation. For the efficiency of the 

 method it is of importance that the metal surfaces should have the 

 greatest purity possible, as small impurities can already cause Volta- 

 potential differences of the order of magnitude of the small counter 

 potential. The coppei- used had been cauterized with nitric acid 

 immediately before the construction and the sealing in of the apparatus. 

 The whole apparatus was mounted on a pretty large glass foot, as 

 is used for incandescent lamps, so that it could be fused into a 

 glass globe without the metal parts being, heated too much. It was 

 heated at 400° in high vacuum for six hours; after this the copper, 

 even though it was a little tarnished before in a few places, presented 

 a perfectly pure metallic surface. 



In figures 2 to 4 curves are represented as instances of the results 

 of such measurements, which refer to neon of a pressure of 

 0,51 m.m., to a neon-helium mixture of 30% helium and a pressure 



