412 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE 
and for commercial purposes ; but it is decidedly inferior to silver, both on account of 
its tendency to oxidise when heated in the air, and also because it changes weight in 
contact with copper sulphate solution without the passage of an electric current. 
Dr. Gore'" has made observations upon this subject, and our own experience has 
shown that no constancy of weight is to be found under these circumstances. Silver, 
on the other hand, seems to be entirely unaffected by contact with neutral solution of 
the nitrate. 
§ 2. The readiest method of measuring currents is, perhaps, that followed by 
Kohlrausch, both in his earlier! and in his recentJ work upon this subject, viz., 
to refer the current to the earth’s horizontal magnetic intensity (H) with an absolute 
galvanometer. The constant of the galvanometer is readily found from the data of 
construction with the necessary accuracy, and there is no doubt that in a well-equipped 
magnetic observatory the method is satisfactory. But the determination of H is no 
such easy matter, and its continual fluctuations must be registered by an auxiliary 
instrument. Many of the results obtained in past years do not appear to be very 
trustworthy, though Kohlrausch and Wild, who has discussed the sources of error 
in an elaborate manner, are of opinion that a high degree of accuracy is attainable. 
When, however, a current determination is the only object, the exclusion of this 
element seems to be desirable, except for rough purposes, when a sufficiently accurate 
value of H can be assigned without special experiment. 
§ 3 . Of the arrangements which may be adopted for measuring the mechanical 
action between a fixed and a mobile conductor conveying the same current, the one 
that is best known is Weber’s electro-dynamometer.§ Two fixed coils may be 
arranged on Helmholtz’s principle, so as to give at the centre a very uniform field of 
force, in which the movable coil is suspended bifilarly. In the equilibrium position the 
planes of the coils are perpendicular, but under the influence of the current they tend 
to become parallel, and the deflection produced may be taken as a measure of the 
square of the current. The constant of the instrument, so far as dependent upon the 
dimensions of the large coils, can be readily determined ; the difficulty is to measure 
with sufficient accuracy the dimensions of the small coil, and to determine the force of 
restitution corresponding to a given rotation. The latter element is usually obtained 
indirectly from the moment of inertia of the suspended parts and from the time ot 
vibration. If the small coil contain a large number of turns in several layers, its con¬ 
stant is very difficult to determine by direct measurement. If, indeed, we could trust 
to the inextensibility of the wire, as some experimenters have thought themselves 
able to do, the mean radius could be accurately deduced from the total length of 
wire, and from the number of turns; but actual trial has convinced us that fine 
* ‘ Nature,’ Feb. 1, 1883, Feb. 15, 1883. 
f Fogg. Ann., Bd. cxlix., S. 170, 1873. 
. X Ber. der Phys.—Med. Ges. zu Wurzburg, 1884. 
§ Maxwell’s ‘ Electricity,’ § 725. 
