448 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE 
We found 
No. 1 De La Rue = *7510 Clark. 
No. 2 „ = 7512 ,, 
No. 3 „ = 7382 „ 
No. 4 ,, = -7458 ,, 
Mean ,, = 7465 „ 
Mr. De La Rue (Phil. Trans., Vol. 169, Part I.) found a result decidedly smaller, 
the explanation of which is to be sought in the fact that in his experiments the cells 
were making a current of about yyyy ampere, whereas in ours the electromotive force 
is measured when no current passes. 
It may be useful to record also a comparison between our Clark’s and a new form 
of Danif.ll, introduced by Sir W. Thomson. This cell is charged with zinc sulphate 
of sp. gr. P02, and with saturated solution of copper sulphate. The zinc is not 
amalgamated. According to Sir W. Thomson’s directions, the circuit of the cell is 
closed through 250 ohms, and the E.M.F. measured is that between the poles under 
these conditions. After the current had been running for about an hour and a half, 
the E.M.F., which had been increasing, became fairly constant, and its value was then 
743 in terms of Clark No. 1 . The comparison was made on April 8, 1884.* 
§ 33. We now pass to the description of the method adopted for the absolute deter¬ 
minations. The current, after leaving the current-weighing apparatus, is caused to 
traverse a wire of knowm resistance R, whose stout copper terminals rest on the copper 
bottoms of suitable mercury cups H, K (Plate 17, fig. l). To these cups are brought 
also the terminals of the derived branch, in which are included the galvanometer and 
the standard cell. 
On account of the strength of the currents (about y ampere) the resistance required 
to be of special construction in order to avoid too great heating. 
Two ebonite rods were held in a parallel position by a frame of wood, and round these 
uncovered german silver wore was wrapped so as to be exposed to the air as much as 
possible. The rods are about a foot apart, and are grooved, the better to keep the 
wire in its place. The resistance is about 4 B.A., and was determined with the aid of 
a jive and a single .f At 17°'6 its value is 4‘00699 B.A. 
Even this resistance-ware heats sensibly when the current of y ampere is passed 
through it for more than a few seconds. The increment of resistance was determined 
by observations taken immediately after the passage for some minutes of a stronger 
current (about 1 ampere). In this way it was found that for the currents usually 
employed a correcting factor 1‘00041 must be introduced to take account of the heat¬ 
ing, independently of course of the correction necessary for the difference between 
17°'6 and the temperature of the atmosphere at' the time of an absolute determination. 
* See notes. 
t For the methods used to ascertain the value of the five the reader is referred to former papers. 
