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VIII. The Discharge of Positive Electrification by Hot Metals, 

 By the Hon. R. J. Strutt, Fellow of Trinity College, 

 Cambridge *. 



IT was first observed by Guthrie (Phil. Mag. [41 xlvi. 

 p. 273) that red-hot metals were able to discharge posi- 

 tive electrification, and that white-hot metals were able, in 

 addition, to discharge negative electrification. 



The experiments described in the present paper were 

 undertaken with a view to determine at what temperature 

 the discharge of positive electrification became sensible, and 

 how far this temperature was dependent on the conditions of 

 the experiment, namely, the state of the metallic surface and 

 the nature and condition of the surrounding gas. 



It may be well to say at once that I have been able to 

 detect the effect at temperatures of about 270° C, and that it 

 increases with extraordinary rapidity when the temperature is 

 raised. 



The temperature at which the effect becomes sensible does 

 not appear to depend very largely on the nature of the 

 metallic surface, or on the surrounding gas, but it alters 

 nearly as much from slight changes in the condition of the 

 surface of the metal, produced by continued heating, as it 

 does when an entirely different metal is substituted. 



With this preface I will explain the method of experi- 

 menting adopted. 



When it is desired to measure a very small electrostatic 

 leakage there are two alternative methods. Either a very 

 sensitive instrument must be used, the time of leakage being 

 short, or, on the other hand, a less sensitive instrument may 

 be employed, the leakage being allowed to proceed for a 

 longer period. 



The quadrant electrometer may be arranged so that it will 

 detect a very small electromotive force ; on the other hand, 

 its capacity is large, and its zero seldom remains so fixed in 

 position as not to wander very appreciably in, say, one hour. 

 Further, it is necessary to use long connecting wires to the 

 instrument, and these in any case involve loss of insulation, 

 owing to the conductivity existing normally in air (0. T. R. 

 Wilson, Proc. Roy. Soc. lxviii. p. 151). If gas flames are 

 in use at the same time there is all the more chance of 

 leakage from the high-potential connecting wires, because 

 of the conductivity of the gases from the flame. 



A gold-leaf electroscope, read by a microscope with a 

 micrometer eyepiece, is free from the objections of large 

 * Communicated by the Author. 



