"This process differs radically from all the others in forcing through the metal to be heated electrically such volumes of current that its own resistance is sufficient to bring every molecule of the section traversed by the current to the desired temperature." 3. Current is taken from a lighting or power circuit, is stepped down to the required 3 or more volts and higher volume, and is passed through a secondary circuit in which the greatest resistance is offered by the pieces of the metal to be welded. The cross-section and unit resistivity are so proportioned to the flow of current that the resistance produces red or white heat at the point of welding. The hot metals are then forced together and the weld is made. The apparatus necessary are:

1. A generator of alternating current.

2. A step-down transformer, carried in the body of the welder.

3. Apparatus for regulating the current, and sometimes apparatus for automatically shutting off the current as soon as welding heat is reached.

1 "Arc Welding," C. B. Auel, Proceedings of the Engineers' Society of Western Pennsylvania, May, 1909; presented before the Society, April, 1909.

2 Size of hole = 1¾ inches diameter by 1½ inches deep. Size of carbon = 1½ by 6 inches. Time = 3 minutes 30 seconds (includes 45 seconds for reversing plate).

3 Hermann Lemp, The Engineering Magazine, Aug., 1894.

4. Clamps for holding the metal to be welded and to transmit the current to it.

The Thomson process presents a number of decided advantages. Among them:

1. It is at present the best all-around welding machine for welding continuous runs of one weld, such as printers' chases.

2. The power used is claimed to give a 75 per cent, heat efficiency; the power is used only as long as needed, and is turned off as readily as the hot-flame welding burners.

3. The heating is rapid, even, entirely local, and is under control.

4. There is no excessive heating as with the electric arc; hence no excessive oxidation or decarbonizing of the metal.

5. The clamps hold the work in accurate alignment and furnish pressure enough to squeeze well the hot metal.

6. The workman is in no danger of injuring his eyes by excessive light, nor is the current at all dangerous. The operator works without dark glasses or protective apron and can hold the metal bars while the welding is going on.

The present limitations of the process seem to be:

1. Though it will weld odd or job work, it is practically limited to continuous welding of one article, known as repeat welding.

2. Though such metals as brass and cast iron can be welded-on the Thomson machine, the company does not-recommend it for such metals as have a. marked melting point and which are not plastic below that point. High-carbon steel does not give an altogether satisfactory weld with this process.

3. The machine demands power at irregular intervals. For this reason station engineers may object to having a single machine of large size on their lines.

Apparatus And Current. - The Generator

Welding work can be done with current from city lighting circuit or the firms will sell a generator built for the purpose. A machine built for sizes of iron and steel not more than 1/3 inch square may be connected to the city alternating lighting wires at 54 or 104 volts, requiring no transformer. For work larger than 1/3 inch square section it is best to get the alternating generator made for the machine. The following is a table of dynamos especially adapted to welding:

Generators Specially Adapted For Electric Welding, 220 To 3300 Volts

(Warren Electric Mfg. Co.)

In this process alternating current is invariably used, though there is no electrical reason why direct current should not be used. It is claimed for alternating current that its heating action is more uniform. As it flows mostly on the surface of the conductor, its heating effect begins and is most intense on the surface. This heat is evenly conducted to the core of the welded pieces; thus the radiation and conductance are offset. The periodicity of the current may vary between 50 and 250. As low as 20 may be used. Guarini1 recommends 80 to 250 on welds of 4-inch square section, which, however, is larger than work ordinarily welded by electricity. The lower the periodicity, the less will be the skin effect, and hence the less the tendency for the current to crowd toward the outside of the parts to be welded. The Thomson machine is now designed for alternating current of 40 to 60 cycles.

Figure 15 shows the Thomson apparatus diagramatically.