most respects similar to the lamp figured in Fig. 1, but in which the
ends of the two carbons rest against the side of a small cylinder of
fireclay or other refractory material, which is mounted on a
horizontal axis and can be rotated thereon by a worm and worm-wheel
actuated by an endless cord passing over a grooved pulley. In the lamp
one of the carbon-holders is rigidly fixed to the framing of the
apparatus, and the other is mounted on a point so as to enable the
length of the arc playing over the clay cylinder to be regulated by
the action of an electro-magnet attracting an armature in opposition
to the tension of an adjustable spring.

In the same exhibit will be found specimens of Mr. Hedges' two-way
switches, which have been designed to reduce the tendency to sparking
and consequent destruction which so often accompanies the action of
switches of the ordinary form. The essential characteristic of this
switch, which we illustrate in elevation in Fig. 3 and in plan in Fig.
4, lies first in the circular form of contact-piece shown in Fig. 4,
and next in the fact that the space between the two fixed
contact-pieces is filled up with a block composed of compressed
asbestos, the surface of which is flush with the upper surfaces of the
two contact-pieces. The circular contact-piece attached to the switch
lever can be turned round so as to present a fresh surface when that
which has been in use shows indications of being worn, and a good firm
contact with the fixed contact-pieces is insured by the presence of a
spiral spring shown in the upper figure, and which, owing to an error
in engraving, appears more like a screw than a spring. In order to
prevent bad connection through dust or other impurities collecting
within the joint, the electrical connection between the fulcrum of the
switch lever and the circular contact-piece is made through the bent
spring shown edgeways in Fig. 3.--_Engineering._