284 report— 1879. 



since the last Report of the Committee on Electrical Standards, September 4, 

 1867. 



At that time it was thought worth while to issue a provisional unit of capacity 

 to meet the requirements of the electric telegraph service, in the shape of a mica- 

 paraffin condenser of capacity to be determined by the ballistic method; but 

 according to the Report no decision had then been arrived at whether the new unit 

 should be issued by the Committee or on Professor Jenkin's own responsibility. In an 

 appendix to that Report, Professor Jenkin has published the results of several deter- 

 minations of the capacity of a certain condenser of Mr. Latimer Clark's construction, 

 adjusted to equal 10 _u electro-magnetic absolute units ; in the values he obtained 

 there was an approximation of only - 42 per cent, between the mean and any single 

 result, so that copies of such a preliminary standard would probably not have been 

 correct within much less than 1 per cent. 



This apparent variation of capacity was due chiefly to the absorption of charge 

 by the dielectric used to separate the conducting plates ; and, I believe, this con- 

 denser, and others made like it at the time, have failed altogether in insulation since. 



In 1869 I carried out a series of experiments for Messrs. Clark & Muirhead, to 

 determine the best and most durable dielectric to use in the construction of con- 

 densers for the electrical testing and working of submarine telegraph cables, and I 

 soon found that a given dielectric absorbed less, the freer it was from foreign 

 matter. The materials that seemed best suited for the purpose were paper, mica, 

 paraffin, and shellac. Several condensers were made in the manner described by 

 Professor Jenkin in Appendix IV. to the above-mentioned Report, in which these 

 materials were used, purified in different ways, great care being taken in every 

 case to prevent the deposition of moisture on the plates during the construction of 

 the condensers. The plates that showed the least amount of absorption, with ordi- 

 dinary differences of potential, were those made of mica coated with shellac, that 

 had been purified with absolute alcohol. The condenser which I now exhibit was 

 constructed of such plates, and its capacity adjusted to equal one-third of a micro- 

 farad on comparison with four condensers lent to me by Mr. Latimer Clark and 

 Mr. Forde, one of which was the condenser referred to above in Professor Jenkin's 

 report, and the other three were made from it by Messrs. Laws and Lambert, 

 assistants to Mr. Latimer Clark. According to my own determinations at the time, 

 by the ballistic method, using a needle whose period of vibration was 8 seconds, its 

 value was -331 microfarad, and I decided to take as correct the mean - 332 of this, 

 and the value - 333 got by comparison with the four condensers referable to Pro- 

 fessor Jenkin's determinations. Ever since then, copies have been made of this 

 condenser and supplied by Elliott Bros., the late firm Warden & Co., and Clark, 

 Muirhead & Co. to manufacturers of scientific apparatus and others as standards ; 

 the number so issued is over 600. From the close agreement of the determinations 

 just made by Mr. Hockin (a member of the late Committee on Electrical Standards), 

 of the capacity of this condenser with mine made nine years ago (vide his note ap- 

 pended to this), it will be seen that probably no change greater than one-third 

 per cent, has taken place in it, and, therefore, one might confidently recommend the 

 British Association either to adopt this form of condenser as their temporary 

 standard, or to appoint another committee to re-investigate the subject. I might 

 add that I found very little absorption in condensers of brass plates embedded in 

 paraffin wax allowed to solidify under pressure, and also in some made with silvered 

 glass plates embedded in the same manner. In the absolute determination of the 

 capacity of these condensers by discharge through a galvanometer, no greater 

 differences than one-third per cent, need be made in the results, even with needles 

 varying in period of vibration by as much as from 2| seconds to 25 seconds. This 

 is the result of experience ; and in defining the capacity it will be sufficient for all 

 practical purposes to specify complete saturation of the condenser and the method 

 of measurement adopted. 



I have asked Professor Ayrton to draw the attention of the Section to the fact 

 that there is no one authorised to certify to the correctness of copies of the 

 various electrical standards originated by the British Association. I would sug- 

 gest that some public body, such as the Kew Observatory, be asked to undertake 



