352 
LORD RAYLEIGH ON THE CONSTANT OF MAGNETIC 
Clark cell itself being treated as a standard of E.M.F., by which to determine the 
value of the current, which traversed the known resistance, and also the helix by 
which the magnetic rotation was produced. The arrangements differed so little from 
those elaborately described in the paper referred to, that it seems unnecessary to enter 
into the matter at length. If the reader will refer to Plate 17, fig. 1, he will 
understand the electrical connexions, and he may suppose the current-measuring 
apparatus EGF, replaced by the magnetising helix. In point of fact this helix was 
situated in another room at a distance from the E.M.F. compensator and its galvano¬ 
meter T. The direction of the current in the helix was reversed by a mercury key 
of the rocker pattern, and care had to be taken that at this moment the galvano¬ 
meter contact Q was open. The general nature of the arrangement will be sufficiently 
understood when it is said that the want of balance between the E.M.F. of the 
Clark and that at the terminals of the resistance II was made up by E.M.F., taken 
from an auxiliary circuit, the value of which was afterwards expressed in terms of the 
Clark. Denoting the force thus added or subtracted by r, upon a scale according to 
which the force of the Clark was p, the actual difference of potential at the terminals 
of II may be written 
(l i X Clark. 
17. As it was intended to use currents of about one ampere, the resistance R 
was made about [ 1 J] ohms. The construction was somewhat similar to that of the 
[4] described in § 33 of the former paper, but on account of the increase in the current 
to be carried, three wires of German silver were used in parallel. The amount of 
heating was unimportant for the purposes of the present investigation. 
The value of the [l-|] was determined by comparison with a combination of three 
standard units, one (taking the whole current), and two in parallel (giving the ^). 
At 13° the resistance is D4945 B.A. At 15°, which was adopted as the standard 
temperature for R and for the Clark, we have 
R = 1-4958 B.A. 
18. In consequence of the heating of the copper wires, the current (usually obtained 
from secondary cells) fell off somewhat rapidly during a set of observations, and it was 
found convenient to take readings of the E.M.F. compensator simultaneously with 
the adjustment of the polarimeter. The former readings were taken by myself and 
the latter by Mrs. Sidgwick, while the flame fat which the optical observer should 
not look) was regulated by an assistant, who also recorded the circle readings. 
The procedure will be most easily explained by an example, for which purpose I 
take at random the observations of July 25, recorded in Table II. 
