On Photometric Paddle-Wheels. 
657 
1910—11.] 
on which the writer’s apparatus is based obviates the necessity for this, 
and permits of making the only moving part in one solid piece. 
This rotating part, which is somewhat similar to a paddle-wheel in 
appearance, may take various forms, of which the simplest is shown in 
fig. 1. It consists of two flat triangular vanes V and V' fixed on the shaft of 
an electric motor C, by means of which they are rotated. The motor is fixed 
to a circular base E, which turns on a vertical pivot D ; and when in use 
the apparatus is placed so that the pivot is directly underneath the beam 
of light L. In spinning round, V and V' interrupt the light, and the amount 
of interruption evidently depends on the azimuth of the base, which is 
ascertained from a circular scale round its edge, or in any other^convenient 
way. It is clear that this angular measurement, on which the light 
intensity depends, can be made with any required degree of accuracy. 
The following is a List of Symbols , with their Meanings : — 
a. The azimuth angle through which the motor is turned about its vertical pivot. 
The zero position is that in which the axle of the paddle-wheel is parallel to 
the beam of light ; and rotation of the motor counter-clockwise about the pivot, 
looking down upon the latter from above, is considered to give a positive 
value to a, and vice versa. 
a m . That maximum value which it has been decided that a shall not exceed. 
c. A constant for each paddle-wheel ; which may be defined as half the distance 
between two imaginary planes of indefinitely great area, perpendicular to the 
axle of the paddle-wheel, and so placed, one in front of the wheel, and the 
other behind, as to just be in contact with the extreme tips of the vanes : 
e.g. for the paddle-wheel shown in fig. 1, c = J fg. 
E. The “ Eclipse- Angle,” i.e. the angle through which the paddle-wheel turns 
while the particular ray of light considered is being intercepted by a vane 
(or pair of vanes, if there be two sets of vanes one behind the other). 
vol. xxxi. 42 
