In an electric circuit the ends of two pieces of metal brought together
form the point of greatest resistance in the electric circuit, and the
abutting ends instantly begin to heat. The hotter this metal becomes, the
greater the resistance to the flow of current; consequently, as the edges
of the abutting ends heat, the current is forced into the adjacent cooler
parts, until there is a uniform heat throughout the entire mass. The heat
is first developed in the interior of the metal so that it is welded there
as perfectly as at the surface.

[Illustration: Figure 42.--Spot Welding Machine]

The electric welder (Figure 42) is built to hold the parts to be joined
between two heavy copper dies or contacts. A current of three to five
volts, but of very great volume (amperage), is allowed to pass across
these dies, and in going through the metal to be welded, heats the edges
to a welding temperature. It may be explained that the voltage of an
electric current measures the pressure or force with which it is being sent
through the circuit and has nothing to do with the quantity or volume
passing. Amperes measure the rate at which the current is passing through
the circuit and consequently give a measure of the quantity which passes in
any given time. Volts correspond to water pressure measured by pounds to
the square inch; amperes represent the flow in gallons per minute. The low
voltage used avoids all danger to the operator, this pressure not being
sufficient to be felt even with the hands resting on the copper contacts.

Current is supplied to the welding machine at a higher voltage and lower
amperage than is actually used between the dies, the low voltage and high
amperage being produced by a transformer incorporated in the machine
itself. By means of windings of suitable size wire, the outside current may
be received at voltages ranging from 110 to 550 and converted to the low