ON STANDARDS OF ELECTRICAL RESISTANCE. 497 



instruments already made, when adjusted to Miil these conditions, may be 

 trusted to measure the difference of potentials produced by* single cell of 

 Daniell's to an accuracy of a quarter per cent. • It must be remembered that 

 the constancy of value of the unit of each instrument depends not only on 

 the constancy of the potential indicated by the gauge, but also on the con- 

 stancy of the force in the field traversed by the suspended needle. As both 

 these may be expected to decrease gradually from year to year (although very 

 slowly after the first few hours or weeks), rigorous methods must be adopted 

 to take such variations into account, if the instrument is to be trusted to as 

 gh-ing accurately comparable indications at all times. The only method 

 hitherto provided for this most important object consists in the observation 

 of the deflection produced by a measured motion of one of the quadrants by 

 the micrometer-screw («) when the four quadrants are put in metallic com- 

 munication with one another through the principal electrodes — -the force 

 producing this deflection when the potential of the jar is constant ; and there- 

 fore, the jar being brought to one constant potential by aid of the gauge, the 

 amount of the deflection will show whether or not the force of the magnetic 

 field has changed, and will render it easy at any time to adjust the strength 

 of the magnets, if necessary, to secure this constancy. But to attain this ob- 

 ject by these means, the three quadrants not moved by the micrometer-screw 

 must be clamped by their fixing-screws so that they may be always in the 

 same position. 



§ 16. The al)solute constancy of the gauge cannot be altogether relied upon. 

 It certainly changes to a sensible degree with temperature, and to very dif- 

 ferent degrees, and even in different directions, as will be seen (§ 32) in con- 

 nexion with the description of the portable electrometer to be given later. 

 But this temperature variation does not amount in ordinary cases probably 

 to as much as one per cent. ; and it is probable that after a year or two any 

 further secular variation of the platinum torsion sj)ring will "be quite insen- 

 sible. It is to be remarked, however, that secular experiments on the elas- 

 ticity of metals are wanting, and ought at least to be commenced in our 

 generation. In the meantime it ^dll be desirable, both on account of the 

 temperature variation and of the possible secular variation in the couple of 

 torsion, to check the gauge by accurate measurements of the time of oscilla- 

 tion of the needle with its appurtenances. The moment of inertia of this 

 rigid body, except in so far as it may be influenced by oxidation of the metal, 

 of which I have as yet discovered no signs, may be regarded as constant, 

 and therefore the amount of the directing coviple due to the magnets may be 

 determined with great accuracy by finding the period of an oscillation when 

 the four quadrants are put in connexion through the charging rod with the 

 metal mounting bearing the guard plates, &c. I have not as yet put into 

 practice any of the obvious methods, founded on the general "principle of 

 coincidences used in pendulum observations, for determining the period of the 

 oscillation ; but although not more than twenty or thirty oscillations can be 

 counted, it seems certain that with a little trouble the period of one of them 

 piay be determined without much trouble to an accuracy of about -j-L per cent. 



Ajjsoitjte Elecirometer. 

 § 17. The absolute electrometer (fig. 11, Plate VI.) and the other instru- 

 ments of Class III. are founded on a method of experimenting introduced by 

 Sir Wm. Snow Harris, and described in his first paper " On the Elementary 

 Laws of Electricity"* thirty-foiir years ago. In these experiments a con- 

 * Philosophical Transactions, 18-31. 

 1867. 2 L 



