494 Prof. Fleming and Mr. Clinton on the 



capacity is independent of the time of charging. This 

 method has been extensively employed, the only difference 

 being in the nature of the commutator for charoino- and 

 discharging the condenser. Maxwell suggested the use of a 

 tuning-fork as a switch, the vibrating prongs being furnished 

 with a stylus which made and broke contact with a mercury 

 cup, or vibrated -to and fro between two fixed contacts. The 

 great objection to this arrangement is the uncertain 

 duration of the contact. Hence other experimentalists have 

 employed an electrically maintained tuning-fork with a 

 vibrating contact used as a means of driving synchronously 

 an electromagnetic contact-breaker, which in turn makes a 

 better and longer contact between two stops alternately. A 

 device of this latter kind was successfully employed by 

 Professor J. J. Thomson *, Dr. G-lazebrook f, and Professors 

 Fleming and Dewar % in various experiments connected 

 with the measurement of specific inductive capacity at low 

 temperatures. 



We have had considerable experience with all these tuning- 

 fork devices, but we have found them troublesome in practice, 

 especially when a large number of measurements have to be 

 made. In addition there is always an uncertainty as to the 

 actual duration of the contact, which makes it impossible to 

 determine by calculation whether the condenser is being 

 fully discharged at each vibration. After a great many 

 preliminary experiments, we finally devised the following 

 appliance, which when properly made never fails to give 

 satisfaction and renders the measurement of small capacities, 

 even as small as one ten-thousandth of a microfarad, a matter 

 as easy as the measurement of resistance on a THieatstone 

 bridge. 



The instrument as constructed consists of a continuous 

 current electric motor of £ HP., but for certain purposes, 

 and where very small capacities have to be measured, it is 

 preferable to employ a motor of J HP. This motor is bolted 

 down upon a baseboard and has connected with it a starting 

 and regulating resistance. The motor is preferably 100 or 

 200 volt shunt wound motor. To the shaft of this motor is 

 connected by a flexible coupling the commutating arrangement, 



* See J. J. Thomson, Phil. Trans. Roy. Soc. 1883, p. 718, "On the 

 number of Electrostatic Units in the Electromagnetic Unity of Elec- 

 tricity/' 



t Also R. T. Glazebrook, Phil. Mag. Aug. 1884, vol. xviii. p. 98, " On 

 a Method of measuring the Capacity of a Condenser " ; or Proc. Brit. 

 A^soc. Leeds, 1890. 



\ J. A. FlemiDg and J. Dewar, Proc. Roy. Soc. London, 1896, vol. lx. 

 p. 368, "On the Dielectric Constant of Liquid Oxygen and liquid Air." 



