se. Glow-lamps usually take
current of 110 or 250 volts pressure.

In arc lamps fed with direct current the tip of the positive carbon has
a bowl-shaped depression worn in it, while the negative tip is pointed.
Most of the illumination comes from the inner surface of the bowl, and
the positive carbon is therefore placed uppermost to throw the light
downwards. An alternating current, of course, affects both carbons in
the same manner, and there is no bowl.

The carbons need frequent renewal. A powerful lamp uses about 70 feet of
rod in 1,000 hours if the arc is exposed to the air. Some lamps have
partly enclosed arcs--that is, are surrounded by globes perforated by a
single small hole, which renders combustion very slow, though preventing
a vacuum.

ELECTROPLATING.

Electroplating is the art of coating metals with metals by means of
electricity. Silver, copper, and nickel are the metals most generally
deposited. The article to be coated is suspended in a chemical solution
of the metal to be deposited. Fig. 84 shows a very simple plating
outfit. A is a battery; B a vessel containing, say, an acidulated
solution of sulphate of copper. A spoon, S, hanging in this from a glass
rod, R, is connected with the zinc or negative element, Z, of the
battery, and a plate of copper, P, with the positive element, C. Current
flows in the direction shown by the arrows, from Z to C, C to P, P to
S, S to Z. The copper deposited from the solution on the spoon is
replaced by gradual dissolution of the plate, so that the latter serves
a double purpose.

[Illustration: FIG. 84.--An electroplating outfit.]

In silver plating, P is of silver, and the solution one of cyanide of
potassium and silver salts. Where nickel or silver has to be deposited
on iron, the article is often given a preliminary coating of copper, as
iron does not make a good junction with either of the first two metals,
but has an affinity for copper.

[17] From the Latin _commuto_, "I exchange."

[18] Only the "drum" type of armature is treated here.

[19] This refers to continuous-current dynamos only.

[20] The Robertson.

Chapter X.

RAILWAY BRAKES.

The Vacuum Automatic brake--The Westinghouse air-brake.

In the early days of the railway, the pulling up of a train necessitated
the shutting off of steam while the stopping-place was still a great
distance away. The train gradually lost its velocity, the process being
hastened to a comparat