Current from a lighting circuit of 54 or 104 volts can be carried directly to the welding clamps without being transformed. Such a welder is called a direct welder, and is used for small work only. With current of a 220- or 440-volt circuit a transformer is necessary. Besides which, it is safest and cheapest to use a current of not more than 4 to 10 volts in any event.

1 Scientific American Supplement, Nov. 5, 1904.

The transformer used is of the core type. It consists of a core of soft iron surrounded with the primary and secondary coils, the first of which introduces the primary current at relatively high voltage and low amperage, and the second of which leads off the current for the secondary or welding circuit at relatively low voltage and high amperage. The secondary coil is a solid copper casting encircling the core. In the larger machines, the heating effect of the current transformation is overcome by passing a steady current of oil over the transformer which is encased in a tight box. This oil is either air- or water-cooled.

Fig. 15. - Diagram of Thomson electric welder.

Figure 16 shows a machine with two transformers, one for heating and one for welding. The machine is described as follows:

"There are two separate transformers in the machine, on one of which is mounted a gun-metal platen, sliding on the right-hand or welding transformer. The left-hand contact, or electrode, is located between the contacts of the heating transformer and is adjustable for any required space between the electrodes of the two transformers by a screw at the left of the welder; the right-hand platen being moved to and from the left-hand contact by the pressure lever at the right. The cam levers, which hold the piece to be welded tightly on the electrodes, are fastened to the cast-iron bracket, and are adjustable to varying thicknesses of stock.

"The circuit in the transformers is opened and closed by two pole break-switches, which are furnished with the welder and should preferably be installed at the back of the machine; treadles, which are connected by chains to the break-switches, project under and at the front of the welder, and are operated by foot.

"One piece is laid on the terminals of the heating transformer in the direction of front to back of the welder, and is securely held by bringing forward the cam lever, the circuit is then closed by placing the left foot on the break-switch lever, and, while the piece is rapidly heating between the electrodes, the other piece is laid on the electrodes of the right-hand er welding transformer and tightly clamped by the other cam lever. The foot is then released from the break-switch treadle of the heating transformer and transferred to that of the welding transformer, when the second piece, immediately coming to a welding heat, is forced against the heated section of the first piece by the pressure lever, upsetting against and fusing with it. The foot is then removed from the break-switch treadle, and the cam levers are thrown back, releasing the welded pieces." held by bringing forward the cam lever, the circuit is then closed by placing the left foot on the break-switch lever, and, while the piece is rapidly heating between the electrodes, the other piece is laid on the electrodes of the right-hand er welding transformer and tightly clamped by the other cam lever. The foot is then released from the break-switch treadle of the heating transformer and transferred to that of the welding transformer, when the second piece, immediately coming to a welding heat, is forced against the heated section of the first piece by the pressure lever, upsetting against and fusing with it. The foot is then removed from the break-switch treadle, and the cam levers are thrown back, releasing the welded pieces."

Fig. 16. - Thomson double transformer electric welder, for dash and fender frames. The clamping device can be modified to take other right-angle and of welds.

The transformer is the heaviest part of the welding machine. So it is placed in the body of the welder frame, underneath the clamping table. It thus gives stability to the machine. Figure 16 shows the transformers in plain sight. In figure 21 the transformer is covered from view in the body.

Typical Thomson Welders