70 
ME. HARRIS ON THE TRANSIENT MAGNETIC STATE 
substance, or otherwise to a ring- of lead. This disc has smooth rounded edges, 
and is balanced on a fine central point A of hardened steel, in such way that 
the centre of gravity and centre of motion coincide as nearly as possible ; the 
whole is sustained on a small agate cup, set in a short cylindrical piece of brass 
a, fig. 4, and screwed, when required, into the shoulder of the brass cylinder 
which passes through the round hole of the pump-plate as represented in fig. 6. 
The sliding rod n e above mentioned is in this case withdrawn for a short di- 
stance within the cylinder. The disc is set in motion at the rate required by 
means of a line wound rapidly off it, from a train of wheels ; it is then covered 
with a screen, and with a receiver, as in fig. 6, and the exhaustion made as per- 
fect as possible ; in which case it will continue to revolve for a considerable 
time. 
A simple or compound magnet is put into rapid rotation in a similar way, 
being previously mounted on a ring of lead ; besides straight bars, a compound 
magnet was employed for this purpose, having its extremities turned up in a 
perpendicular direction for about an inch, as in c d, fig. 7? and occasionally a 
circular disc of magnetised steel, fig. 5. The former has its point of support 
fixed in a transverse bar of brass a b, fig. 7, which projects in a circular hole 
drilled through the centre of the bars.* 
6. When it became necessary to measure with great precision the rapidity 
of rotation, the machine represented in fig. 3 & 8 was employed. It consists 
of a metallic disc M kept in motion by a delicate train of wheels and pinions 
/; p . The rapidity of the rotation being accurately measured by an index x, 
moving on a small circle divided into twenty parts. The weight W, fig. 3, 
which by its descent keeps up the rotation, falls in a tall narrow receiver r r', 
supported against the under surface of the pump-plate. 
A circular hole is drilled at s' through the plate for the silk cord to pass, 
from the pulley at 3/ ; by means of this hole also, the receiver r r' is exposed 
to the action of the pump. The machine is wound up and set in motion by 
means of the brass rod n e, fig. 3, already described (4), without disturbing the 
state of the exhaustion ; the extremity of the rod being formed into a key. 
The revolving disc is screened by glass or any other substance, over which 
* This method of obtaining a rapid rotation in an exhausted receiver, suggested itself on perusing 
the account of Mr. Serson’s horizontal speculum in the Royal Society’s Transactions. 
