160 Royal Society . — 



instance, extreme shortness in the standard or tested conductor), only 

 the shortest possible duration of current is allowable, the position of 

 the galvanometer, with reference to the battery and the other portions 

 of circuit, must he so arranged that its needle may show no sensible 

 deflection when the key is pressed to the middle position. Ignorant 

 or inadvertent operators are probably often led into considerable 

 mistakes in their measurements of resistance by confounding de- 

 flections due to direct electro-magnetic influence of batterv, bat- 

 tery electrodes, or standard, tested, or testing- conductors, on the 

 needle of the galvanometer, with the proper influence of a cur- 

 rent through its own coil, — a confusion which can only be resolved 

 by making or breaking the galvanometer circuit while the battery 

 circuit is kept made, for which there is no provision in the ordinary 

 plans of Wheatstone's halance. We may, however, suppose that 

 most experimenters will be sufficiently upon their guard against 

 error from such a source. But there is another and a much more 

 important advantage in the double-break arrangement which I now 

 propose. Electro-magnetic inductions will generally be sensible* in 

 some or in all of the different branches of the compound circuit, 

 and cannot, except in very special cases, be exactly balanced as 

 regards electromotive force between P and Q with the arrangement 

 which makes an exact balance of resistances. Hence, at the moment 

 when the battery contact is made, there must generally be an electro- 

 motive impulse between Q and P, which will drive a current through 

 the galvanometer coil, and make an embarrassing deflection of the 

 needle if the galvanometer circuit is complete at that instant (as it is 

 in the common plans of Wheatstone's balance), and will require the 

 observer to wait until the needle comes to rest, or until he can tell 

 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 galvanometer 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 



* I make them as little sensible as possible in my coiled testing-conductors, 

 and in sets of coiled standards of resistance, by either doubling each coil or each 

 branch of each coil on itself, or by reversing the lathe at regular intervals hi 

 winding on any single coil on a bobbin,— a plan which has also the advantage of 

 rendering the direct electro-magnetic action of any coil so wound very small or 

 quite insensible on any galvanometer needle in its neighbourhood. 



