294 Mr. E. W. B. Gill on the Electrical 



light internally reflected. The light, after passing through 

 the prism, was brought to a focus by the second quartz lens 

 D, exactly similar to the first, at the vertical slit G, which 

 was the entrance to the detector F to be described later. It 

 was found better in practice to have the spark-gap vertical 

 and not horizontal as shown in the figure. The tubes carrying 

 the lenses D, D were telescopic, allowing the lenses to be 

 focussed for light of any refractive index within certain 

 limits. The apparatus was mounted on a strong graduated 

 spectroscope table which allowed the prism and the receiver 

 to be rotated through any desired angle. The rays in 

 travelling from the spark-gap S to the detector F traversed 

 a distance of about 90 cm. of air (at atmospheric pressure) r 

 and the central ray passed through about 3 cm. of quartz, 

 The tubes were blackened on the insides to prevent light 

 being reflected from the sides, but as it seemed uncertain 

 whether lampblack absorbed all the rays the sides of the 

 tubes were cut away in places and two diaphragms C, C r 

 inserted to stop oblique light. 



The Electrical Measurements. 



The light which entered the detector fell upon a zinc plate 

 and caused a number of negative ions to be set free at its 

 surface. The charge which thus escapes from the plate is 

 very small since the slits at G and _B (fig. 2) are narrow, so 

 that in order to detect the effect of the light it is necessary 

 to magnify the charge. 



Professor Townsend has shown that if the zinc plate be 

 arranged opposite and parallel to a second plate at distance d , 

 and if the electric force between the plates is X, then if n^ 

 ions be set free at the zinc plate the total number of ions n 

 reaching the positive plate is given by the equation 



where « and ft are functions of X and the pressure p onlv. 



The multiplying factor \*—EJJlL can be made as 



a. — p e^~ p > a 



large as we please by suitably adjusting X and p. During 

 an experiment p was kept at 4 mm. of mercury, and X was 

 about 700 volts per cm. This gave a big factor and was not 

 too near the sparking potential to produce irregularities. 

 The charge transferred was measured by a quadrant electro- 

 meter, and the ratio of the readings of the electrometer gave' 

 in consequence the ratio of the numbers of ions set free for 



