Rail welding was first suggested by Thomson and practised with one of his machines. On account of the importance of electric rail welding and the special apparatus needed, the process will be described in detail.

The most recent application of the Thomson process is to the welding of sheets of metal. Two sheets of steel are lapped and welded at regular intervals in points similar to riveting. "The method consists in bringing two pointed electrodes against the two sheets, or prepared sheets with slight projections, by indenting or punching. The current flows through these projections which are pressed down flat on the sheet, effecting the weld. This method is used in interior furnishings of steel railway cars, passenger coaches, steel furniture, sheet-metal ware, etc."1

Fig. 29. - Thomson specimen. Automobile clincher rim.