Besides its use as a welder, the machine may be used as a preheater of metals to be brazed or bent. It will sometimes be preferable to braze or solder a joint, when the two metals cannot be allowed to lose their shape or have any of their substance pressed into an upset: the welder can then be used as a preheater. The current would be regulated to bring the metals to a slightly lower-than-welding heat and keep them at this heat. In brazing brass, this is the best-known method of preheating, because a torch preheater always burns out some of the zinc in the brass and oxidizes the copper.

The Thomson welder may be used to anneal spots in armor plate. This is done by connecting the positive to the armor plate and pressing the negative clamp against the spot to be annealed.

Tests

In general, tests of electric welds show that from 75 to 95 per cent, of the original strength of the metal is reached. In cases where the upset is not cut off, the strength can be increased above 100 per cent. Welds of low-carbon steel and low sulphur-and-silicon iron, if well made and worked or drawn after working, will approximate 100 per cent, in strength.

It is sometimes asked if the electric current does not damage the metal. Electric welding is no more harmful to the metal than any other process. In fact, the control of the heat is so exact and overheating and reheating so seldom happen, that electric welds run uniformly high in tensile and elastic strength. A "burned" weld seldom occurs - the oxid at the joint is forced out into the upset and ground off. It may be emphatically stated that the electric-resistance welds are the best yet made. As an instance, such a misused and overstrained utensil as a printers' chase seldom gives at the weld.

Sir Frederick Bramwell1 states that 1 1/8-inch round bars can be welded in 2 1/4 minutes with an average tensile strength of 91.9 per cent, against four minutes' time and 89.8 per cent, strength when smith-welded.

The results of a series of tests of electrically welded metals carried on at the Watertown Arsenal2 may be abridged as follows:

Twenty-nine broke at the weld.

Seventeen within 2 inches of the weld.

Eleven within the range of moderate heat.

Two near the grips.

1 "Elec. Engineering Formula," p. 673.

2 Transactions of the American Society of Mechanical Engineers, 1889, p. 97.

Welds of wrought iron were 5 to 10 per cent, below unit strength; fracture fibrous or slightly spongy.

Welds of steel were from 50 to 80 per cent, less than unit strength.

Copper welded at 5 to 10 per cent, less than unit strength.

Steel welded to wrought iron at about the strength of the iron.

Brass gave an uncertain weld with wrought iron and had a strength at the weld of 8 1/2 to 16 1/2 tons to the inch.

Steel welded with German silver with a strength of 20 tons to the inch.

Some welds of steel were about unit strength and some of iron were above unit strength.

A number of these bars had upsets, however, and the upset does not seem to have increased the strength very much.

When electric welding was first tried out there was serious complaint that the welds were burnt, spongy, and weak. This was due to the fact that the metals were melted together and were not worked. The welding machines with automatic swage blocks prevent crystallization at the weld, as does also hammering after welding. The weld is still liable to be weak on the edge of the heating radius. Many joints that will hold at the weld will break an inch either side because the heat has destroyed the properties of the metal.