ON STANDARDS OF ELECTRICAL RESISTANCE. 173 
The complete expression for R is therefore 
Bi pe 
R=a5 5381455817307 3055°5. 
The nature of the electrical action in the experiment may be stated as 
follows :— 
Suppose the plane of the coil to coincide with magnetic north and south, 
and that the coil is revolving in the direction of the hands of a watch. Then 
the north side of the coil is moving from west to east, and therefore expe- 
riences an electromotive force tending to produce an upward current. The 
south side of the coil is moving from east to west, and therefore there is a 
tendency to produce a downward current in it. If the circuit is closed there 
will be a current upwards on the north side, and downwards on the south side 
round the coil. 
Now this current will tend to turn the north end of the suspended magnet 
towards the east. But the earth’s magnetic force tends to turn it towards 
the north, so that the actual position assumed by the magnet must depend on 
the relation between the strength of the current and the strength of the 
earth’s magnetism. But the strength of the current depends only on the 
velocity of rotation, the resistance of the coil, and the strength of the earth’s 
magnetism. Hence the position of the magnet will not depend on the strength 
of the earth’s magnetism, but only on the velocity and the resistance of the 
coil. 
We must remember that the coil in its revolution comes into other posi- 
tions than that which we have mentioned. As the north side moves towards 
the east, the current continually diminishes till it ceases when it is due east. 
The current then commences in the opposite direction with respect to the 
coil ; but since the coil itself is now in a reversed position, the effect of the 
current on the suspended magnet is still to turn the north end to the east. - 
The action of the current on the magnet is therefore of an intermittent 
nature, and the position of the magnet is not fixed, but continually oscillating. 
The extent of these oscillations, however, is exceedingly small. In fact, if T 
be the time of vibration of the magnet from rest to rest under the action of 
the earth, and if ¢ be one quarter of the time of revolution of the coil, and if 
8 be the deviation as read on the scale, then the same amplitude of these 
oscillations will be 
?? 
C= é. 
In the actual experiment poabout ae and 6 less than 400 millimetres, 
so that the whole extent of vibration would be less than ;1, of a millimetre 
on the scale. This vibration was never observed and did not interfere with 
the distinctness of vision. 
The only oscillations observed were the free oscillations of the magnet. 
They arose from accidental causes at the beginning of the experiment, and 
were subject to slight alterations in magnitude due to changes of speed of 
rotation, the passage of iron steamers in the Thames, &c. The time of one 
vibration was about 9-6 seconds, and by reading the scale at the extremities 
of every vibration a series of readings was obtained, the intervals between 
which were approximately equal. 
Now since the deviation is proportional to the velocity 
