140 
MR. W. CROOKES ON THE ILLUMINATION OF LINES OF 
ELECTRICAL RADIOMETERS. 
502. Successful observations were ultimately obtained by making radiometers with 
metallic vanes, in the following manner :— 
The needle point on which the fly rotates is connected with a copper wire, a, 
fig. 10, passing down the central tube and attached to the platinum terminal, b, sealed 
Fig. 10. 
through the glass. The other terminal, c, passes through the top of the bulb, and 
is of aluminium wire inside. The vanes are of aluminium, lampblacked on one side, 
and connected by aluminium arms to a hard steel cup ; the whole fly is therefore in 
metallic connection with the lower terminal. 
503. At a moderately low pressure, the coil being attached and the fly being made 
the negative pole, a faint halo appears on the bright side of the vanes, and there is a 
tendency to negative rotation. As the exhaustion proceeds, the dark space appears and 
gradually -widens out. The space on the black side is nearly double the thickness of 
that on the bright side, but is fainter (491); on exhausting still more, the dark space 
widens out until it forms a large, irregular shaped, oval round each vane. Whilst the 
dimensions of this oval are small, the fly does not move definitely one way or the other, 
but when the dark space on the lampblacked side has grown till its luminous boundary 
touches the glass, positive rotation commences, and continues with increasing speed as 
the exhaustion increases. The difference in size between the dark spaces on each side 
of the vanes being but small, that from the bright side soon impinges against the glass, 
and prevents the positive rotation from acquiring much speed. 
504. Another instrument was therefore made similar to the one last described, 
but having one side of each metallic vane covered with a thin piece of mica. It was 
connected with the coil, and experiments tried as exhaustion proceeded. 
At a pressure of 4’5 millims. a halo of a velvety violet appearance forms on the 
metallic side of two of the vanes only, the others being dark. The movement is of a 
negative tendency. 
