160 SCIENTIFIC APPARATUS. 



is determined, not by the want of perfect insulation, but by the 

 low potential of the source from which it is* electrified, the 

 same method of increasing the quantity of electricity received 

 by it can be employed ; and by removing the second and 

 oppositely charged conductor, after the conductor to be charged 

 has been disconnected from the source, the potential of the 

 latter may be raised so as greatly to exceed that of the source. 

 Apparatus with a movable second conductor arranged for use 

 in this way, are called electrical condensers. The condenser seems- 

 to have been first described by Volta in 1782. 



It will be seen from what is said above that quantity of electri- 

 city, capacity, and difference of potential are so related that if in a 

 given case two of the three are known, the third is at once 

 determined. Consequently the instruments required for mea- 

 suring these quantities cannot be separated from each other. 

 Among the most important may be mentioned absolute standards 

 of capacity, that is, insulated conductors whose capacity is known 

 from their dimensions the simplest is a metallic sphere at a great 

 distance from any other conductors ; relative standards of 'capacity \ 

 or accumulators whose capacity has been determined in terms of 

 a known absolute standard; accumulators, or condensers whose- 

 capacity can be varied at will, and by known amounts. Examples 

 of these are afforded by Sir William Thomson's platy meter and 

 various adjustable accumulators formed by the combination of 

 two or more separate accumulators. Other instruments required 

 in connection with these are electrometers for absolute and relative 

 Measurements of differences of potential, and standards of difference 

 of potential, such as the standard element of Mr. Latimer Clark. 



Connected with the measurement of capacity is the study of the 

 condition of an insulating medium which separates two oppositely 

 electrified surfaces. It was first shown by Faraday (1837) that 

 the capacity of an accumulator depends not merely upon the size 

 and conformation of the conducting surfaces, but also on a pro- 

 perty of the insulating medium between them which he called its 



