MISCELLANEOUS PRACTICAL APPLICATIONS. 



341 



tip which stops the current abruptly. The layer of oxygen then 

 collects as a bubble and rises, leaving the platinum tip again in 

 contact with the acid when another rush of current takes place, 

 and so on. From 200 to 1,500 interruptions per second maybe 

 produced by this arrangement according to the size of the plati- 

 num tip, the inductance of the circuit and the value of the electro- 

 motive force. 



41. Electric welding. Thomson's process. The two metal 

 rods to be welded are connected to the terminals of an electric 

 generator and brought into contact with each other. The cur- 

 rent, flowing across the 

 relatively high resistance 

 contact, heats the ends of 

 the rods to the melting 

 temperature, the rods are 

 then pushed slightly to- 

 gether and the weld is 

 complete. Alternating cur- 

 rent is generally used in 

 this welding process ; a 

 transformer takes current 

 at high voltage from ordinary supply mains and delivers a very 

 large current at very low voltage to the rods to be welded. 



The wet process. When a direct-current generator having an 

 electromotive force of from 200 to 500 volts is connected to an 

 electrolytic cell with small cathode, the cathode becomes intensely 

 heated. This effect is utilized for welding as follows : The two 

 rods a and b, Fig. 43, which are to be welded are connected 

 to the negative terminal of the dynamo D. The positive ter- 

 minal of the dynamo is connected to a metal nozzle from which 

 a jet of salt water issues. This jet impinges upon the ends of 

 the two rods and quickly fuses them together. 



Fig. 43. 



