696 
LORD RAYLEIGH OH THE VALUE OE THE BRITISH 
It will be seen that the agreement is practically perfect, the coefficient given by the 
extreme speeds suiting also the requirements of the intermediate speeds. The maxi¬ 
mum difference corresponds to about yfyths of a millimetre only in the deflections of 
the principal magnetometer. The number 23*612 X 10 9 is therefore to be regarded as 
the resistance in absolute C.G.S. measure of the platinum-silver standard at 13°. If, 
however, the correction be rejected, the result will be different by decidedly less than 
one part in a thousand. 
Although the experiments of the second series will not bear comparison with those 
of the third, it may be well to mention that they lead to substantially the same con¬ 
clusion. The simple mean (taken with exclusion of August 29) of all the values is 
23*633, and after introduction of the correction proportional to the square of the 
speed, 23*618. 
The results of the first series of spinnings are given in Table VIII. They have been 
reduced by Dr. Schuster, so as to show the value of the platinum-silver standard in 
absolute measure from the observations of each night at each speed. The mean radius 
of the coil was taken from the first measurements, and a somewhat higher value of U 
was employed than the subsequent calculation of the ring currents seemed to justify. 
Table VIII.—First series. 
Teeth. 
80 
60 
45 
35 
30 
Resistance in absolute measure of standard at 13°. 
23*651, 
23*632, 
23*628 
23*646, 
23*629, 
23*601 
23*678, 
23*691, 
23*686 
23*608, 
23*615, 
23*632 
23*644, 
23*639, 
23*628 
Mean 23*637 
Mean 23*625 
Mean 23*685 
23*665 Mean 23*630 
Mean 23*637 
23*643 
Comparison with the standard B.A. units. 
Four distinct sets of comparisons between the platinum-silver standard and the 
ultimate B.A. units have been effected in the course of these investigations, and two 
distinct methods have been followed. In the first method two coils of about five units, 
called for brevity [5]’s, were compared separately with five standard units combined in 
series with mercury cups. Secondly, the two [5]’s in series were compared with a [10]. 
Thirdly, the [10], the two [5]’s, and four singles were combined in series and compared 
with the platinum-silver standard [24]. The differences in every case were expressed 
in divisions of the wire of Fleming’s bridge, whose value in terms of the B.A. unit 
is known. This method is simple enough in principle, but the arrangement of so many 
mercury connexions is troublesome, and the calculation of the innumerable tempera¬ 
ture corrections necessary is tedious. The labour would have been greater still had we 
not been able to avail ourselves of the previous work of Professor Fleming, who had 
carefully compared the various standard units, and had drawn up a chart on which is 
exhibited the comparative resistances of the coils over a considerable range of tempera- 
