246 Sir David Salomons on the Electric 



I do not intend to deal with the last three periods ; but in 

 regard to the first, although many interesting facts were 

 brought to light, the subject does not appear to have been 

 systematized, nor am I able to find a great deal published on 

 the subject before the period of Professor Crookes, perhaps 

 for the reason that no one has taken the pains to collect all 

 the information and publish it together. 



Many important discoveries were no doubt made and the 

 facts published in some obscure quarter, where they probably 

 remain to this day. The general interest which now exists 

 in science and the large technical Press were non-existent 

 but a very few years ago. 



I now propose to describe briefly a number of points of 

 interest in connexion with vacuum-tubes and some classes of 

 work which may be performed by their use, the outcome of 

 work extending over twenty-five years. 



It appeared to me that two points required settling in 

 regard to these tubes, which, however, are not completely 

 exhausted as the name would suggest; viz. : — 



(1) "That the number of bands produced in a given tube 

 should be brought under control, that is to say, that the 

 conditions under which they are formed should be solved." 



(2) "That the reason for their existence should also be 

 found out." 



In regard to the first point, I believe that this question is 

 solved and the results given in a paper by me published in 

 the ' Proceedings of the Royal Society/ in volume lvi. 



The origin of the bands has been surmised by many of the 

 early workers, in fact they have assumed that they are pro- 

 duced in consequence of self-inductive effects ; but I have 

 not been able to discover any proof that such is the case. I 

 shall be able to show by experiments to be described that this 

 view is correct ; and from the methods adopted, which are 

 probably the only means available for the purpose, give the 

 clue why the first investigators did not advance beyond a 

 state of conjecture. 



The tubes I employed contained rarefied air and various 

 other gases ; but I found for the particular class of experi- 

 ments that rarefied air sufficed in all cases, and that it was 

 not necessary to proceed to very high exhaustion. 



The great efforts made by earlier investigators in trying to 

 obtain enormous electromotive force appeared to me wrong 

 for the class of experiment they were entering upon. I 

 therefore tried the opposite course — one which has proved so 

 successful in mathematics, viz. the reduction of the leading 

 factor to the smallest quantity possible, — and observed what 



