. Sheet metal workers' manual; a complete, practical instruction book on the sheet metal industry, machinery and tools, and related subjects, including the oxy-acetylen welding and cutting process . ed between heavy copper conductors ofa welder, becomes heated in attempting to carry the largevolume of current. The degree of heat depends on theamount of current and the resistance of the conductor. In an electric circuit the ends of two pieces of metalbrought together form the point of greatest resistance inthe electric circuit, and the abutting ends instantly be-gin to heat. The hotter this met
. Sheet metal workers' manual; a complete, practical instruction book on the sheet metal industry, machinery and tools, and related subjects, including the oxy-acetylen welding and cutting process . ed between heavy copper conductors ofa welder, becomes heated in attempting to carry the largevolume of current. The degree of heat depends on theamount of current and the resistance of the conductor. In an electric circuit the ends of two pieces of metalbrought together form the point of greatest resistance inthe electric circuit, and the abutting ends instantly be-gin to heat. The hotter this metal becomes, the greaterthe resistance to the flow of current; consequently, as theedges of the abutting ends heat, the current is forced intothe adjacent cooler parts, until there is a uniform heatthroughout the entire mass. The heat is first developedin the interior of the metal, so that it is welded there asperfectly as at the surface. 351 352 SHEET METAL WORKERS MANUAL The electric welder (Figure 283) is built to hold theparts to be joined between two heavy copper dies or con-tacts. A current of three to five volts, but of very greatvolume (amperage), is allowed to pass across these dies,. Figure 283.—Electric Spot Welding Machine. and in going through the metal to be welded, heats theedges to a welding temperature. Voltage and Amperage.—It may be explained that thevoltage of an electric current measures the pressure orforce with which it is being sent through the circuit andhas nothing to do with the quantity or volume measure the rate at which the current is pass-ing through the circuit, and consequently give a measureof the quantity which passes in any given time. Volts ELECTRIC WELDING 353 correspond to water pressure measured by pounds tothe square inch; amperes represent the flow in gallonsper minute. The low voltage used in this electric welderavoids all danger to the operator, this pressure not beingsufficient to be felt even with the hands restin
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