ON STANDARDS OF ELECTRICAL RESISTANCE. 171 
In this expression the quantities Dnlaare determined before the experi- 
ment is made. The only quantities to be observed are T, the number of 
seconds in 100 revolutions, and 6, the deviation in millimetres of the scale. 
Part [V.—Derarzs oF THE EXPERIMENTS, 
In the experiments at King’s College, June 1863, 
m, the number of windings, was 307. 
1, the effective length of wire, 302-063 metres. 
sin’ a=1—-+021756. 
D, the distance from the mirror to the scale, 2-9853 metres. 
Determination of Velocity. 
A wheel of 100 teeth turned by an endless screw caused a bell to be struck 
every 100 revolutions of the coil. The times of the bells, as observed with 
a chronometer, serve to determine T. 
Determination of Deviation. 
5 is the difference between the reading of the scale when the magnet is 
acted on by the earth only, and when it is acted on also by the induced cur- 
rents in the coil. ‘To determine 6, the reading of the scale is made when the 
coil is at rest, or when the circuit is broken, Another reading is taken with 
the connexion complete and the coil in motion. If the earth’s magnetism 
remains the same, the difference of these readings is the true value of 3; but 
since the direction of the earth’s magnetic action is continually varying, we 
must find the difference of declination between the times of the two readings, 
and calculate what would have been the undisturbed reading of the scale at 
the time when the deviation was observed. 
In our experiments this correction was made by comparison with the pho- 
tographic registers of magnetic declination made at Kew at the same time 
that our experiments were going on. 
Corrections. 
The corrections being small may be taken separately. Each has to be 
multiplied by the factor already considered, 
pa 2002" Dalsin’ 447 444 B4+O04DFE+4+F+6+H + he}. 
Té 
A. Correction for the dimensions of the section of the coil. 
2 Pal A a, 
A=te4e ‘ = sin? a@cos? a—e* sin? @ 
B. Correction for level. Let the axis of rotation be inclined to the vertical 
at an angle measured towards the north, and let the angle of the dipping- 
needle with the horizontal be I, then there will be a correction, 
B= —tan I sin f. 
In the actual experiment the level was taken with a spirit-level reading to 
12", and found correct to at least that degree of accuracy. 
C. Correction for the induction of the suspended magnet on the coil. The 
strength of the magnet, as compared with that of the magnetic field, was mea- 
