VALUE OF THE BRITISH ASSOCIATION UNIT OF RESISTANCE. 
319 
and perforated symmetrically along a circle with holes equally numerous with the 
armatures. When all is regular, the prongs of the fork are seen in one phase only, so 
long as the eye retains a position fixed in space. 
When the wheel runs lightly, independent driving power may be dispensed with, a 
sufficient amount of work being obtainable from the intermittent governing current. 
In the present case the whole apparatus, consisting of the two forks and the wheel, 
was driven by one current supplied from three Grove cells. The only difficulty 
experienced is in starting the wheel. By means of string passed once round the shaft, 
alternately tightened for the advance and slackened for the return, it is easy to cause 
the wheel to achieve a speed in excess of the necessary eight revolutions per second. 
But it will not usually happen, every time the speed falls through the proper value, 
that the wheel will engage with the fork. For this purpose it is necessary that at the 
moment in question the phase of the wheel should be correct, within limits, which may 
be narrow when there is no great margin of power; and this can only happen by 
chance. Several attempts may be necessary before success is reached. With a little 
practice, however, there is no great loss of time, the ear learning to recognise, by the 
gradual slowing and subsequent quickening of a sort of beat, when the wheel has 
passed through the right speed without engagement. A fresh impulse is then given 
without waiting further. After a start is once effected, the wheel will usually run, 
keeping perfect time with the fork, until the battery is exhausted. 
The wheel employed in the experiments we are now concerned with, has four soft 
iron armatures, and is governed by the interrupter-fork of frequency 32. The speed 
of the wheel is thus eight revolutions per second; and a single hole in a paper disc 
carried round with it allows eight views of the pendulum per second, the smallest 
number of views obtainable by direct use of the fork being 32. Altogether we 
may regard the frequency of the interrupter-fork as being multiplied four times 
precisely in the frequency of the auxiliary fork, and as divided four times precisely 
in the frequency of the wheel. The former is directly comparable with the standard 
fork, and the latter with the clock. The standard fork was screwed to the table 
precisely as during the electrical measurements. A thermometer placed between 
the prongs gave the temperature with fair accuracy. 
The calculation of the results is very simple. Supposing in the first instance that 
the clock is correct, let a be the number of cycles per second (perhaps 4 ^) between the 
wheel and the clock. Since the period of a cycle is the time required for the wheel to 
gain, or to lose, one revolution upon the clock, the frequency of revolution is 8 ffi;a. 
The frequency of the auxiliary fork is precisely 16 times as great, i.e., 128ff;16«. If 
b he the number of beats per second between the two forks, the frequency of the 
standard is 
128±16a±6 
To give an idea of the magnitudes of the numbers concerned, it will be advisable to 
