670 
LORD RAYLEIGH OH THE VALUE OF THE BRITISH 
manner as when the throw is taken. In all galvanometers, however, a very sensible 
damping remains in operation even when the circuit is open, of which the greatest 
part is doubtless due to aerial viscosity ; and it is certain that the retarding force 
arising from viscosity is not simply proportional to the velocity at the moment, 
without regard to the state of things immediately preceding. 
In particular, the force acting upon the suspended parts as they start suddenly 
from rest in the observation of the throw, must be immensely greater than in subse¬ 
quent passages through the position of equilibrium, when the vibrations have assumed 
their ultimate character. I calculate that in the first quarter vibration (i.e., from the 
position of equilibrium to the first elongation) of a disc vibrating in its own plane and 
started impulsively from rest, the loss of energy from aerial viscosity would be 1 -373 
times that undergone in subsequent motion between the same phases. From this it 
might at first appear that in this ideal case the logarithmic decrement observed in the 
usual manner would need to be increased by more than a third part in order to make 
it applicable to the correction of a throw from rest; but in order to carry out this view 
V 
consistently we should have to employ in the formula the time in which the needle 
would vibrate if the aerial forces were non-existent instead of the actually observed 
time of vibration. Now since the action of viscosity is to increase the time of vibra¬ 
tion, the second effect is antagonistic to the first, so that probably the error arising 
from the complete neglect of these considerations is very small. 
There is another point in which it appears to me that the theory of the ballistic 
galvanometer is incomplete. It is assumed that the magnetism of the needle in the 
direction of its axis is the same at the moment of the impulse as during regular vibra¬ 
tions. Can we be sure of this \ The impulse is due to a momentary but very intense 
magnetic force in the perpendicular direction, and it seems not impossible that there 
may be in consequence a temporary loss of magnetism along the axis. If this were so, 
the actual impulse and subsequent elongation would be less than is supposed in the 
calculation, and too high a value would be obtained of the resistance of the secondary 
circuit in absolute measure. In making these remarks I desire merely to elicit dis¬ 
cussion, and not to imply that Rowland’s value is certainly four parts in a thousand 
too high. 
Determinations of the absolute unit have been made also by H. Weber/'' whose 
results indicate that the B.A. unit is substantially correct. In the absence of sufficient 
detail it is difficult to compare this determination with others, so as to assign their 
relative weights. 
The value of the B.A. unit in absolute measure is involved in the two series of 
experiments executed by Joule on the mechanical equivalent of heat.f The result 
from the agitation of water is 24868, while that derived from the passage of a known 
absolute current through a resistance compared with the B.A. unit was 25187. The 
* Phil. Mag., Jan., Feb., March, 1878. 
t Phil. Trans., Part II., 1878. Brit. Ass. Rep., 1867 ; Reprint, p. 175, 
