327 



precisely to what point its oscillations converge, the current being 

 kept flowing all the time, before he can discover whether the balance 

 of resistances has been attained or not. This absolutely precludes 

 very refined testing, since, whether by the heating and consequent 

 augmentation of resistance of some part of the balanced branches, or 

 by thermo-electric reactions consequent on heating and cooling effects 

 at junctions of dissimilar metals when the branches of the balance 

 are not all of one homogeneous metal, or last, though not least, by 

 the eye losing the precise position where the galvanomer needle or 

 indicating image rested, it is not possible to use the full sensibility 

 of the galvanometer for testing a zero if its needle is allowed to re- 

 ceive such a shock in the course of the weighing. Embarrassment 

 from this source is completely done away with by using the double 

 spring key described above, and giving time, from its first to its 

 second contact, to allow the electro-magnetic induction to subside. 

 An extremely small fraction of a second is enough in almost all cases ; 

 and the operator may therefore generally press the key home almost 

 as sharply as he will or can. But when there is a large "electro- 

 dynamic capacity"* in any part of the balance-circuit, as, for ex- 

 ample, when the coil of a powerful electro-magnet with soft ironf 

 core is the conductor whose resistance is tested, it may be necessary 

 to keep the key in its middle position for a few seconds before press- 

 ing it home, to avoid obtaining what might be falsely taken for an 



* TMs term I first introduced in a communication " On Transient Electric 

 Currents" (Phil. Mag., June 1853), to designate what for any electric current 

 through a given conductor is identical in meaning with the " simple-mass equi- 

 valent" in the motion of Attwood's machine as ordinarily treated. A rule for 

 calculating the electrodynamic capacity is given in that communication ; also 

 the rule, with an example, in Nichol's Cyclopaedia, article " Magnetism Dy- 

 namical Relations of." 



t Giving a resistance to the commencing, to, the ceasing, or to any other 

 variation in the strength of an electric current (precisely analogous to the effect 

 of inertia on a current of common fluid), which it seems quite certain must be 

 owing to true inertia (not of what we should at present regard as the electric 

 fluid or matter itself flowing through the conductor, but) of motions accom- 

 panying the current, chiefly rotatory with axes coinciding with the lines of mag- 

 netic force in the iron, air, and other matter in the neighbourhood of the con- 

 ductor, and continuing unchanged as long as the current is kept unchanged. 

 SeeNichol's Cyclopaedia, article " Magnetism Dynamical Eelations of," edition 

 1860 ; also Proceedings of the Royal Society, June 1856 ; or Phil. Mag., vol. 

 Jan.- June 1857. 



