high resistance to the current, but is not
destroyed by the resulting heat.

In Fig. 80 is shown a fan propelling liquid constantly through a pipe.
Let us assume that the liquid is one which develops great friction on
the inside of the pipe. At the contraction, where the speed of travel
is much greater than elsewhere in the circuit, most heat will be
produced.

[Illustration: FIG. 80.--Diagram to show circulation of water through a
pipe.]

In quite the early days of the glow-lamp platinum wire was found to be
unreliable as regards melting, and filaments of carbon are now used. To
prevent the wasting away of the carbon by combination with oxygen the
filament is enclosed in a glass bulb from which practically all air has
been sucked by a mercury pump before sealing.

[Illustration: FIG. 81.--The electrical counterpart of Fig. 80. The
filament takes the place of the contraction in the pipe.]

The manufacture of glow-lamps is now an important industry. One brand of
lamp[20] is made as follows:--First, cotton-wool is dissolved in
chloride of zinc, and forms a treacly solution, which is squirted
through a fine nozzle into a settling solution which hardens it and
makes it coil up like a very fine violin string. After being washed and
dried, it is wound on a plumbago rod and baked in a furnace until only
the carbon element remains. This is the filament in the rough. It is
next removed from the rod and tipped with two short pieces of fine
platinum wire. To make the junction electrically perfect the filament is
plunged in benzine and heated to whiteness by the passage of a strong
current, which deposits the carbon of the benzine on the joints. The
filament is now placed under the glass receiver of an air-pump, the air
is exhausted, hydro-carbon vapour is introduced, and the filament has a
current passed through it to make it white hot. Carbon from the vapour
is deposited all over the filament until the required electrical
resistance is attained. The filament is now ready for enclosure in the
bulb. When the bulb has been exhausted and sealed, the lamp is tested,
and, if passed, goes to the finishing department, where the two platinum
wires (projecting through the glass) are soldered to a couple of brass
plates, which make contact with two terminals in a lamp socket. Finally,
brass caps are affixed with a special water-tight and hard cement.

ARC LAMPS.

In _arc_ lighting, instead of a contraction at a point in the circu