the Ions in Gases at Low Pressures. 551 



sion as the distance between the plates AB and CD, was 

 withdrawn from between its poles. The coil was care- 

 fully placed so as to occupy the same part of the magnetic 

 field as that occupied by the space between AB and CD 

 when the magnet was used to affect the rate of leak of 

 electricity between AB and CD. In this way the intensity 

 of the magnetic field between the poles of the magnet was 

 determined for a series of values of the current through the 

 magnetizing-coils of the electromagnet ranging between 1 

 and 4' 5 amperes, and a curve was drawn which gave the 

 magnetic force when the magnetizing-current (observed by 

 an amperemeter) was known. 



The pressure of the gas in the tube containing the plate was 

 reduced by the mercury-pump to 1/100 of a millim. of mercury. 

 As the mean free path of hydrogen molecules at atmospheric 

 pressure and 0° C. is 1*85 X 10~ 5 centim. (Emil Meyer, 

 Kinetische Theorie der Gase, p. 142), and of air 10 -5 centim., 

 the mean free paths of these gases at the pressure of 1/100 

 of a millim. of mercury are respectively 14 and 7*6 millim., 

 and are consequently considerably greater than the greatest 

 distance, 4 millim., through which the electrified particles have 

 to travel in any of the experiments. These are the free 

 paths for molecules of the gas ; if, as we shall see reason to 

 believe, the actual carriers of the negative electrification are 

 much smaller than the molecules, the free paths of these 

 carriers will be larger than the numbers we have quoted. 



The rate of leak of negative electricity to CD when AB 

 was exposed to ultra-violet light was measured by a quadrant- 

 electrometer. The zinc plate was connected with the negative 

 pole of a battery of small storage-cells, the positive pole of 

 which was put to earth. One pair of the quadrants of the 

 electrometer was kept permanently connected with the earth, 

 the other pair of quadrants was connected with the wire 

 gauze CD. Initially the two pairs of quadrants were con- 

 nected together, the connexion was then broken, and the 

 ultra-violet light allowed to fall on the zinc plate ; the nega- 

 tive charge received by the wire gauze in a given time is 

 proportional to the deflexion of the electrometer in that time. 

 By this method the following results were obtained : when 

 the difference of potential between the illuminated plate and 

 the wire gauze was greater than a certain value, depending 

 upon the intensity of the magnetic force and the distance 

 between AB and CD, no diminution in the deflexion of the 

 electrometer was produced by the magnetic field, in fact in 

 some cases the deflexion was just a little greater in the mag- 

 netic field. The theory just given indicates that the deflexion 



2Q2 



