ELECTRICAL MEASUREMENT. 249 



exhibited below. It is well known now to many electricians, and a 

 descriptive pamphlet also accompanies it. I really do not know, con- 

 sidering that the British Association report on electrometers has been 

 republished in connection with the whole series of their reports, that I need 

 go into detail with respect to any of these instruments. This is the 

 very first portable electrometer, and I will tell you how it came into 

 existence. I had one that I was very proud of, I am ashamed to say in 

 these days. I was proud of its smallness, and how easily it could be 

 carried up to the top of Goatfell and back ; and there was one before 

 then, the highest character of which was, that it was heavier than a rifle; 

 but, that was in the days of what Lord Palmerston called the "rifle fever," 

 and I was touched a little with that at the time, being a rifle volunteer ; 

 and I found that my electrometer weighed a pound less than my weapon. 

 It only weighed thirteen Ibs., and the rifle weighed fourteen Ibs. 

 I had that at Aberdeen, but it is not now to be found, although it has 

 been searched for, or it would have been exhibited. Part of it, the 

 stand that was on the top of it, is below. The next that followed, was 

 this one. I got down the weight to about one-half, and I was perfectly 

 satisfied then, and this one has gone up the Goatfell a great many times ; 

 but it is fully described in my book, and in the paper I have referred 

 to. I was showing it with great pride on one occasion to Professor 

 Tait, and I said to him : " You should get one like that." He said, 

 " I will wait until you can get one that you can put into your pocket. 

 Get one the size of an orange, and the'n I will have it." That literally 

 was the origin of this electrometer. I felt rather challenged by what 

 he said, and in the course of my next run up to Glasgow, Mr. White, 

 who is so indefatigable in making new things, and who has so admirable 

 an inventive capacity, helped me in my endeavour, and we had some- 

 thing like this one. In the course of a month, this very electrometer 

 was got into action. This is the first attracted disc electrometer. It 

 differs from the portable electrometers now known merely in some 

 minor details ; the moveable disc turns round with a micrometer screw 

 instead of moving up and down in a slide. In all other respects, it is 

 the same, except the awkward arrangement for placing the pumice, 

 which with my great care, did not lead to any accident, but with 

 almost any other person led to the instrument being destroyed 

 by the sulphuric acid placed on it getting shaken down into the 



