686 
LORD RAYLEIGH ON THE VALUE OF THE BRITISH 
Measurement of tan fi. 
This is the tangent of the angle through which a suspended magnetic needle would 
be turned when the principal magnet is presented to it at a distance j-ff 2 ) to 
the east or west, the axis of the principal magnet lying east and west. Actual 
measurements with the aid of the auxiliary magnetometer were made in April, June, 
and November; and as a check upon the constancy of the magnetic moment frequent 
observations were taken of the time of vibration. 
To explain the procedure it will be sufficient to take the data of the November 
measurement. Two positions were chosen for the principal magnet, nearly equi¬ 
distant from the suspended magnet, to the east and west. The length of the line 
joining the two positions was 695 millims., and it passed horizontally about 36 millims. 
below the suspended magnet. In each position the magnet was reversed backwards 
and forwards several times and readings taken. When the principal magnet was to 
the east, the mean difference of readings due to reversal was 13'55 divisions on the 
millimetre scale. When the principal magnet was in the westerly position, the 
corresponding difference of readings was 14*61. We are to take the mean of these, 
i.e., 14'08, as the difference of readings due to reversal at a distance of 347‘5 millims. 
The half of this, or 7'04, corresponds to the simple presentation or removal of the 
magnet. The distance from mirror to scale was 2670 millims, so that the tangent of 
7‘04 
the angle of deflection was -——. This result has to be adjusted to correspond 
° 2 x2670 1 
with the distance v /(a 2 +6 /2 )| in place of 347‘5. Hence 
tan [i= 
(347'5) 3 x 7-04 
(238 , 5) 3 x 2 x 2670 
= *00408 
In this calculation the error due to the principal magnet having been necessarily 
placed at a different level from that of the suspended magnet is ignored. As a matter 
of fact a relatively considerable correction is required. If 0 be the altitude of one 
magnet as seen from the other, the observed effect is too small in the ratio (1 —3d 2 ) : 1. 
The above written value of tan /a requires to be increased about 3 per cent.; so that 
we take 
tan fi=' 00420. 
Measurement of D. 
For the first and second series of spinnings the distance from mirror to scale was 
measured exactly as described by Hr. Schuster (p. 126 of former paper). The value 
adopted for the second series, after correction for the thickness of the glass window in 
the magnet box, was 
D=2669*0 millims. 
