534 



Dr. Norman Campbell : Further 



as A ; there will be a Volta difference of potential between 

 the electrodes, and the actual P.D. will not be that read on 

 the voltmeter. 



It is possible that a way out of this dilemma might be 

 found by the use of some of the methods described recently 

 by Gompton * in his work on the influence of the Volta effect 

 on the measurement of the speed of the photoelectric electrons. 

 But even those methods were not completely successful, and 

 the results which have been already attained appear to me so 

 conclusive that a further elaboration of the apparatus would 

 not be justified. In any case, it would be very difficult to 

 obtain extremely accurate measurements, for in the neigh- 

 bourhood of V = the current changes so rapidly with V and 

 with the geometrical form of A, that the exact reproduction 

 of a curve is almost impossible. The apparatus is also 

 extremely sensitive to small magnetic fields ; if a piece of 

 soft iron of 30 c.c. volume is placed near the electrode the 

 current for V = is changed about 5 per cent. It is certain 

 that the earth's magnetic field has some influence on the 

 shape of the curves. The quantity i 10 was taken as standard, 

 because saturation appears to be attained with about 10 volts, 

 so that i — i 10 represents the whole current, and io — i v that 

 part of it carried by rays of which the speed is less than V. 

 Since only comparative values are important the choice of a 

 standard is unimportant. 



10. The results of the measurements are given .in Table II. 



Table II. 



P.D. 



Current. 



D same as A. 



D covered with Soot. 



20 



Al. 



•996 



L'000 

 •998 

 •997 

 •996 

 •993 

 •980 

 •959 

 •925 

 •856 

 •710 

 



Brass. 



1-000 

 1-000 

 •997 

 •993 

 •988 

 •984 

 •970 

 •957 

 •894 

 •832 

 ? 

 



Au. 



1-002 

 1-000 

 1-000 

 •999 

 •995 

 •991 

 •975 

 •932 

 •873 

 •777 

 •535 

 



Soot. 



1-016 



1-000 



■991 



•987 

 •972 

 •958 

 •932 

 •907 

 •844 

 •727 

 •509 

 



Al. 



1 1-023 



1-000 



•995 



•988 

 •980 

 •962 

 •928 

 •883 

 •819 

 •679 

 •437 

 



Brass. 



1015 

 1-000 

 ■990 

 ■981 

 •975 

 •960 

 •933 

 •906 

 •830 

 •753 

 ? 

 



Au. 



1-010 

 1000 

 •995 

 •990 

 •986 

 •974 

 •954 

 •913 

 •855 

 •750 

 •510 

 



Soot. 



1-016 



1-000 



•994 



•987 

 •972 

 •958 

 •932 

 •907 

 •844 

 •727 

 •509 

 



10 



9 



8 



7 



6 



5 



4 



3 



2 



1 







1 



* Compton, Phil. Mag. April 1912, p. 579. 



