Prof. Thomson on the Measurement of Electric Resistance. 161 



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 

 indication of too great a resistance to conduction (or " friction al" 

 resistance, as I have elsewhere called it J), being a true indication of 

 resistance or reaction of inertia to the commencement of the current 

 in the electro-magnetically-loaded branch§. In such cases it is im- 

 possible, either by electro dynamic balance or in any other way, to 

 obtain a measurement of resistance without keeping the battery ap- 

 plied for the few seconds required to produce sensibly its final 

 strength of current undiminished by inductive reaction, over and 

 above the time required to get an indication from the galvanometer. 

 But, as already remarked, in all ordinary cases, the inductive reaction 

 becomes insensible after a very small fraction of a second, and the 

 operator may press the double key home to its second contact almost 

 as sharply as he pleases. With such a galvanometer as I have de- 

 scribed, he need not hold it down for more than • 7 of a second (the 

 time of the simple vibration of the needle ||) to test the balance of 

 resistances. The order of procedure will therefore generally be this : 

 — The operator will first strike the key sharply, allowing it to rise 

 again instantly, adjust resistances in the balance-circuit according to 



* This 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 Relations of," edition 

 1860 ; also Proceedings of the Royal Society, June 185G ; or Phil. Mag., vol. 

 Jan.- June 1857. 



J "Dynamical Theory of Heat, Part VI., Thermo-electric Currents," Trans- 

 actions of the Royal Society of Edinburgh, 1851 ; and Phil. Mag. 1856. 



§ It is probable that a Wheatstone's balance, perfectly adjusted for equili- 

 brium of resistances to conduction, and used with the galvanometer circuit con- 

 stantly made, so as to show the whole effect of the inductive impulse, may afford 

 the best means for making accurate metrical investigations on electro-magnetic 

 induction, and especially for determining "electrodynamic capacities" in abso- 

 lute measure. 



|| The mirror galvanometers commonly used in Germany have all much 

 longer periods (ten or twenty times as long in many cases) for the vibration of 

 their needles, and want proportionately longer contacts to obtain full advantage 

 of their sensibility, — in each case a contact during a time equal to that of the 

 vibration of the needle one way being required for this purpose. 



