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The Photo-electric Effect of some Compounds. 891 



Summary. 



1. The object of the experiments was to determine 

 Poisson's Ratio for a steel bar from observations on the 

 distortion of the cross-section. 



2. The first set of experiments indicated that the direct 

 method of measuring the distortion, viz., by attaching 

 mirrors to the two vertical sides of a bent bar, does not give 

 consistent results. 



3. The second set of experiments, in which a more sensitive 

 indicating arrangement was used, gave eight values of a, the 

 lowest being *2909 and the highest '2932. The mean value 

 was *2922 and the maximum difference from the mean was 

 •44 per cent., that is rather less than ^ per cent. 



The experiments described above were carried out in the 

 Physical Laboratory of the University of Bristol. I should 

 like to take this opportunity of thanking Professor Selby of 

 Cardiff for his valuable advice and criticism. 



XCVI. The Photo-electric Effect of some Compounds. 

 By E. S. Willows, M.A., D.Sc* 



IN the Phil. Mag. for September there is an interesting- 

 paper by Dr. A. LI. Hughes with the above title. 

 Among other results Dr. Hughes finds that dry zinc chloride 

 shows no photo-electric effect, but when moist air is admitted 

 to the apparatus relatively large currents are obtained, 

 although water itself is not photo-electrically active. The 

 purpose of the present note is to draw attention to some 

 experiments by Mr. A. E. Garrett, carried out under my 

 direction some years ago, which may throw light on the 

 subject. Mr. Garrett and I had previously found that the 

 halogen salts of zinc emit large quantities of positive and 

 negative ions when they are heated to about 300°. In a 

 second paper (Phil. Mag. June 1907) Mr. Garrett showed 

 that the leak from these salts could be detected at the tem- 

 perature of the room provided they were moist, but if 

 moisture were excluded, little or no current was obtained 

 even when the dry salt was heated to 360°. This, I believe, 

 is the lowest temperature at which thermions have been 

 detected. Dr. Hughes' electroscope is apparently more 

 sensitive, and his method of obtaining the dry salt is certainly 

 more efficient than Garrett's, so that any difference of 

 electrical behaviour in the two states would readily be 



* Communicated by the Author. 



