266 Dr. Wood and Mr. Campbell on the Diurnal 



at first been supposed ; for, imposed upon the progressive 

 change of the ionization, which may continue for some weeks, 

 there is a periodic diurnal variation of the ionization. It 

 was considered advisable to investigate this latter change 

 more completely, abandoning for the present the original 

 purpose of the research. 



The existence of such diurnal variations has been suspected 

 by several observers. Both of the authors*, when conducting 

 separate experiments on spontaneous ionization, have been 

 troubled by inexplicable variations in the current through 

 their vessels ; the current seemed smallest in the afternoon. 

 Jaffef also noticed the variations, and investigated them in 

 some detail, concluding that there was a minimum at 2 p.m. 

 Borgmann % states that the minimum of ionization is at 3 p.m. 

 The first three observers had all worked at Cambridge, but 

 since their experiences agree with those of Borgmann, it is 

 probable that the effects are not due to local causes. A 

 similar diurnal periodicity of the amount of radioactive 

 emanation in the atmosphere has been observed by Dike§. 

 His curve is obtained from the observations on six days only, 

 so that it is impossible to attach much importance to its 

 details, but its principal maximum (1 a.m.) agrees fairly 

 well with one of the maxima of the ionization in our vessels. 



As it was desired to vary the gas in which the ionization was 

 to be measured as well as the metal of which the vessel was 

 made, the first difficulty to be solved was the construction of 

 a vessel which would hold hydrogen or coal-gas for several 

 weeks. In the end the following device was adopted. The 

 vessel, which had a capacity of about 6000 ccs., was placed 

 on a brass plate and covered with a glass bell- jar. The bell- 

 jar was cemented down to the plate with marine glue, and 

 could then be exhausted and filled with any required gas. 

 As the experimental vessel was thus pLiced in an atmosphere 

 of the gas, no precautions were necessary to have it absolutely 

 gas-tight. An insulated electrode passed through the brass 

 plate up into the vessel, and the vessel was kept charged to 

 a constant high potential in the usual way. The arrangement 

 was found to hold a good vacuum for a considerable time. 

 In order to obtain continuous readings of the ionization in 

 the vessel, a device employed by Bronson || was used. The 



* N. B, Campbell, Phil. Mag. April 1905, p. 534 ; A. Wood, ibid. 

 p. 562. 



f G. JafTe, Phil. Mag. Oct. 1904, p. 556. 



j J. J. Borgmann, see ' Science Abstracts/ 1904, No. 1580. 



§ 'Terrestrial Magnetism,' vol. xi. No. 3, p. 128. 



|| H. Bronson, Phil. Mag. Jan. 1906, p. 143. See also 0. Nordman, 

 Comptes JRendus, cxxxYiii. pp. 1418-1420 & 1596-1599. 



