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XI. On a Method of Measuring the Electrical Capacity of a 

 Condenser, and on the Determination by Electrical Observa- 

 tions of the Period of a Tuning-fork. By R. T. Glaze- 

 brook, M.A., F.R.S., Demonstrator of Experimental Physics 

 at the Cavendish Laboratory, Cambridge* . 



THE experiments described in the following paper were 

 undertaken at the request of Messrs. Latimer Clark, 

 Muirhead, and Co., for the purpose of testing one of their 

 condensers " which," quoting from a letter from the firm, 

 " has lately been made and which we use in the factory. In 

 its construction it is the same as all our condensers, and is 

 made of sheets of mica and tinfoil and laid with paraffin -wax." 



The method employed was a modification of one given by 

 Maxwell (vol. ii. § 776), and used by J. J. Thomson (Phil. 

 Trans, iii. 1883) in his recent determination. The following 

 is his description of the arrangement : — 



" In a Wheatstone bridge, ABCD (fig. 1), with the galva- 

 nometer at G and the battery be- 

 tween A and B, the circuit B D is 8 * 

 not closed, but the points B and D 

 are connected with two poles, Ii and 

 S, of a commutator, between which 

 a travelling piece, P, moves back- 

 wards and forwards. P is connected 

 with one plate of a condenser, the 

 other plate of which is connected 

 with D. Thus, when P is in con- 

 tact with S the condenser will be 

 charged, and until it is fully charged 



electricity will flow into it from the battery : this will produce 

 a momentary current through the various arms of the bridge. 

 When the moving piece P is in contact with R, the two plates 

 of the condenser are connected, and the condenser will dis- 

 charge itself through D R ; and as the resistance of D E is 

 infinitesimal in comparison with the resistance in any other 

 circuit, the discharge of the condenser will not send an appre- 

 ciable amount of electricity through the galvanometer. Thus, 

 if we make the moving piece P oscillate quickly from R to S, 

 there will, owing to the flow of electricity to the condenser, be 

 a succession of momentary currents through the galvanometer. 

 The resistances are so adjusted that the deflection of the gal- 

 vanometer produced by these momentary currents is balanced 



* Comrmmicatecl by the Physical Society. Read June 28, 1884. 



