﻿98 Lord Rayleigh on the Experimental 



There are, however, one or two matters as to which doubts 

 may arise. Thus it is essential that the commutator by whose 

 action the condenser is periodically charged and discharged, 

 should introduce no electromotive force on its own account. 

 A more serious doubt hangs over the behaviour of the 

 galvanometer. It is assumed that this instrument indicates 

 exactly the mean current, whether the current be steady or 

 intermittent. The principal error to be feared, arising from 

 a somewhat oblique position of the needle and its temporary 

 magnetization under the condensed charging currents, would 

 be eliminated by reversing the battery. But is it certain 

 that the axial magnetization remains constant^ even when this 

 axis is strictly perpendicular to the magnetic forces due to 

 the currents * ? 



Another question relates to the leads connecting the con- 

 denser with the remainder of the apparatus. These must 

 themselves have capacity, and the effect is easily allowed 

 for t if the capacity is definite. It is here that a doubt 

 arises. Consider for example the coaxial cylinders of the 

 Cambridge condenser. When the condenser is to be in 

 action, a leading wire is brought into contact with the inner 

 surface of the inner cylinder. A rupture of this contact 

 throws the condenser out of action ; but whatever be done 

 with the end of the lead, its electrical situation is not the 

 same as before. It is only in very special cases, if at all, that 

 capacities can be added by simply making contacts. 



Passing on to the condenser itself, we may notice that in 

 almost all cases it has been necessary to provide a guard- 

 ring, on the principle first introduced by Lord Kelvin. This 

 leads to complications, though perhaps not very serious ones. 

 Thomson and Searle have shown how to allow for the guard - 

 ring in the calculation of electrostatic capacity, and how to 

 connect it with the bridge in the electromagnetic measurement. 

 It is a further slight complication that the potential is not 

 quite the same for the guard-ring and for the main part of 

 the condenser. 



It has occurred to me that a condenser, not very different 

 from the Cambridge one, may be so arranged as not only to 

 dispense with the guard-ring, but also to eliminate all questions 

 connected with the capacity of the leads. The principle 

 is that of the variable condenser described in Maxwell's 



* It is possible that the difficulty arising from the uncertain behaviour 

 of steel magnets might be obviated by the use of a galvanometer of the 

 so-called d'Arsonval type. The string galvanometer of Einthoven (Drude, 

 Ann. xii. p. 1059, 1903) would appear to be specially suitable. 



t Thomson and Searle, he. tit. 



