ON AN IMPBOYED FORM OF HIGH INSULATION KEY. 29 



or 2055 feet, and tlie temperature which he found at this depth (75-°) is 

 within less than a degree of the temperature which would be calculated 

 from the observations now reported. 



The Committee have to express their regret at the loss of two of their 

 colleagues — ProP. Clerk Maxwell, and Prof. Ansted — by death, during 

 the past yeai'. 



Report of the Committee, consisting of Dr. 0. J. Lodge (Secretary), 

 Professor W. E. Ayrton, and Professor J. Perry, apfjointed for the 

 ■purpose of devising and constructing an improved form of High 

 Insulation Key for Electrometer Work. 



In the construction of the key it was considered desirable to secure as 

 fiar as possible the following conditions : — 



1. That the insulation should be nearly perfect. 



2. That the conductors should have a very small electrostatic capacity. 



3. That they should be entirely protected from all external induction 



by a metal case. 



4. That the hand of the operator should work the moving parts from 



the outside of the case, so as neither to act inductively on the 

 conductors, nor to electrify insulators by friction. 



5. That there should be no friction whatever between insulators and 



conductors in the moving pai'ts. 



6. That all the insulating parts should be easily removable occasionally 



for cleaning purposes. 



7. That the commercial price of the key should not be unreasonably 



high. 



In the original form of the key the conductors were platinum wires 

 suspended inside a metal case by silk threads, the leading wires being 

 brought to them through large holes in the case. It was found, however, 

 that this arrangement was rather too delicate and troublesome for general 

 use, and it was impossible to artificially dry the air in the case because of 

 the large holes in it. 



It was determined, therefore, to abandon silk strings and to use rigid 

 supports for the conductors, and to allow the conductors to protrude 

 through small holes in the case, so that the leading wires might not have 

 to enter the case to reach them. 



For the supports it was ultimately decided to use, not ebonite, but glass, 

 as the latter is more easily cleaned, and in a dry atmosphere has probably 

 the better insulating power ; moreover it is not liable to contract a coat 

 of acid, which acting on the metal conductors gives rise to a feeble E.M.F. 

 causing some keys to act as extremely weak batteries. 



The insulators are four thin pillars of carefully selected glass, mounted 

 in the case in such a way that they can be easily taken out and cleaned 

 occasionally. Brass caps are cemented to the top of each of the pillars, 

 which are so arranged that each cap is near a small hole in the side of the 

 case, and a short thin rod ending in a binding screw is passed through this 

 hole and screwed into each brass cap after they are in position. 



Small ebonite plugs slide on these rods and ordinarily close the holes 

 through which the rods pass, except when pulled out. When very good 



