ON STANDARDS OF ELECTRICAL RESISTANCE. 477 



a more convenient form, overlooking the disadvantage ■which, they all possess 

 of ill-defined capacity. 



These condensers consist of sheets of tinfoil separated by paraffin and 

 paper, a preparation of gutta percha, or mica — three plans adopted by Mr. 

 Varley, Mr. Willoughby Smith, and Mr. Latimer Clark respectively. 



Condensers of this type have been made approximately equal to a knot 

 of some submarine cable, and the rough units thus introduced are gradually 

 creeping into use, although all electricians have been anxious that the Com- 

 mittee should issue a more scientific standard. Under these circumstances, 

 Mr. Jenkin has adjusted a mica-condenser, approximately equal to lO-i^ 

 absolute electromagnetic units. The capacity of this condenser is assumed 

 as that which it possesses after electrification for one minute, and is mea- 

 sured by the discharge through a galvanometer, in the manner usually prac- 

 tised when testing the charge of a submarine cable. The formula for obtain- 

 ing the measurement in absolute units from the throw of the needle is very 

 simple, requiring only observations of the time of oscillation, of a resist- 

 ance in absolute measure, and of a deflection of the galvanometer-needle. 

 All of these observations can readily be made, so that their accumulated 

 error cannot exceed one per cent. ; and for the present purpose this accuracy 

 is sufficient, inasmuch as, when using the condenser, small variations 

 inevitably occur, arising from the residual discharge. While therefore the 

 new provisional unit of capacity has no claim to a high scientific accuracy, it 

 wiU supply a practical want and introduce a unit based on the principles 

 adopted by the Committee, in place of the random measures supplied by a 

 knot of Persian Gulf or Atlantic cable. 



No decision has yet been arrived at whether the new unit shall be issued 

 by the Committee, or on Mr. Jenkin's own responsibility, nor has the price 

 been fixed. 



The experiments by which it has been obtained are given in an Ap- 

 pendix, 



The practical applications of the standard of capacity are important. It 

 will allow the capacity of submarine cables to be universally expressed in 

 comparable figures, and may lead to improvement by the diminution of the 

 specific inductive capacity of the insulator, precisely as the introduction of 

 units of resistance has assisted the improvement in insulation and conduc- 

 tivity. 



The electromagnetic capacity standard will also, by comparison with the 

 electrostatic standard about to be made, furnish one mode of determining 

 the constant called v in previous Eeports, a number of much importance in 

 the theory of electricity. 



The next unit or standard for consideration is that of the difference of 

 potentials or electromotive force in absolute measure, concerning which tlie 

 experiments have been wholly in Sir William Thomson's hands. He reports 

 that he has at last succeeded in constructing a series of electrometers 

 capable of measuring differences of potential ranging from -^ of a DanieU's 

 cell up to 100,000 cells, and that these measurements can all be reduced to 

 absolute units by comparison with one instrument of the series. 



This class of instruments has been created by Sir William Thom- 

 son, who year by year has produced electrometers each surpassing its pre- 

 decessor, both in accuracy and delicacy ; but although those who have had 

 practical experience of the admirable results obtained by these, have for the 

 last two or three years believed that the limit of excellence has been reached. 

 Sir William Thomson has not ceased to invent better and simpler forms, until 



