150 Intelligence and Miscellaneous Articles. 



equilibrium : the equilibrium is ascertained by means of a sensitive 

 galvanoscope. If r be the resistance of the portion of the wire 

 comprised between the straight sections passing through A and B, 

 S the surface covered by the induced wire, a constant peculiar to 

 the coils employed, we have the following condition of equilibrium, 

 which at the same time gives the value of r : — 



«=2irwCS. 



The employment of a frame carrying p turns of wire gives elec- 

 tromotive forces p times as great as those which would be obtained 

 if Lorenz's disk were used ; consequently the effect of thermoelec- 

 tric perturbations becomes negligible. 



It is known that the determination of C is too complicated for 

 its approximation to be easily indicated. I believe that calculation 

 may be altogether dispensed with by operating in the following 

 manner : — If a fixed coil were employed of infinite length in propor- 

 tion to its diameter, C would be known : we should have, exactly, 



0= — =-, d being the mean distance between two turns of the wire. 



Now we cannot construct an infinitely long coil ; but an equivalent 

 result can be obtained. The movable frame is first placed in the 

 centre of a fixed coil of which the length is, for instance, 2 metres, 

 and the points A and B are obtained as was said above. Then, the 

 movable frame remaining in its place, the fixed coil is brought into 

 a second position, which is the extension of the first : a new interval 

 B B f is thus obtained on the wire, immediately following A B ; the 

 small additive segment B B' is the lengthening undergone by A B 

 when the inducing coil is lengthened 2 metres. We can thus 

 lengthen indefinitely the inducing coil by simply displacing one and 

 the same segment ; and we very quickly arrive at segments which 

 can be neglected in comparison with A B. The determination is 

 then finished. 



It will be remarked that this method is one of the most direct ; 

 it requires no calculation for reduction or correction. If the deri- 

 vation-points, such as A and B, are needle-points, the final dis- 

 tance between them is the final residt sought, without any correction. 

 It follows that the control of the method is equally direct. 



2. The above-explained way to determine C experimentally can 

 be applied to other problems besides the construction of the ohm ; 

 it can be employed especially for constructing a tangent-compass or 

 an absolute dynamometer. For this purpose, either a movable 

 magnet or a movable coil is placed in the centre of a fixed coil. 



The constant of the instrument is equal to —, — S, d being the sum 



of the moments of the deflections obtained on successively removing 

 the fixed coil further and further until it reaches infinite distance. — 

 — Comptes Hendus de VAcademiedes Sciences, Dec. 26, 1882, t. xcv. 

 pp. 1348-1350. 



