Association Unit of resistance in absolute measure. 
665 
which we could drive the coil. Even a tap with the finger-nail upon the magnet-box 
produced but a small disturbance. 
No change was required in the arrangements for regulating and determining the 
speed of the coil, which worked, if possible, more perfectly than before, in consequence 
of the greater inertia of the revolving parts. The divided card was, however, on an 
enlarged scale, and the numbers of the teeth in the various circles were so arranged that 
each circle was available for a distinct pair of speeds according as it was observed 
through the slits in the plates carried by the electric fork or over the top of the upper 
plate. The speeds actually used corresponded to 80, 60, 45, 35, and 30 teeth, seen 
through the slits, be., about 127 times per second. 
The greater resistance of the copper coil (23 instead of 4*6) rendered necessary a modi¬ 
fication in the method of making the comparisons with the standard. The whole 
value of the divided platinum-iridium wire on Fleming’s bridge being only - 2 - 0 - ohm, a 
change of temperature in the copper of not much more than a degree would exhaust the 
range of the instrument. To meet this difficulty it was only necessary to add resist¬ 
ances to the copper circuit so as to compensate approximately the temperature varia¬ 
tions, for it is evident that it can make no difference whether the change of resistance 
of the entire revolving circuit is due to a rise of temperature, or to the insertion of an 
additional piece. The platinum-silver standard was therefore prepared so as to have a 
resistance (about 24 ohms) greater than any which we were likely to meet with in the 
copper, and the additional pieces were relied upon to bring the total within distance. 
As at first arranged, the additional resistance was inserted at the mercury cups, instead 
of a contact piece of no appreciable resistance. During the comparison with the 
standard it was transferred to another part of the circuit. 
In the course of May, 1881, a complete series of spinnings were taken, the arrange¬ 
ments and adjustments being (except as above-mentioned) in all respects the same as 
with the old apparatus. Five different speeds were used, and each of them on three 
different evenings. The work of observing was also distributed as before, Dr. Schuster 
taking the readings of the principal magnetometer, and Mrs. Sedgwick the simulta¬ 
neous readings of the auxiliary magnetometer, while I observed the divided card and 
regulated the speed. At each speed on each evening four readings were taken with 
wire circuit closed, two with positive and two with negative rotation, and in like 
manner four readings were taken with the wire circuit open. Observations on the 
zero with the coil at rest were for the most part dispensed with, as it was thought 
that the time could be better employed otherwise ; in fact, the mean of the two not 
very different positions of equilibrium obtained with positive and negative rotation 
when the wire circuit was open, gives all that is wanted in this respect. In the 
actual reductions we only require the difference of readings with positive and negative 
rotations. 
It. was hoped that these observations would have been sufficient, but on the introduc¬ 
tion by Dr. Schuster of the various corrections for temperature, for the beats between 
4 Q 2 
