304 ELECTRICAL MACHINERY 



seen that the primary current as well as the secondary current 

 of such a transformer is practically independent of the load. 

 The explanation of this would require more space than can 

 be afforded here. 



77. Special Types of Transformers. For various pur- 

 poses special transformers are used and we shall describe 

 briefly here the welding transformer, the instrument trans- 

 former, and the testing transformer. 



Welding Transformer. The welding transformer is 

 designed for the purpose of making welded joints by heating 

 the junction of the metal with an electric current. The 

 two pieces to be joined are clamped tightly together and 

 the secondary of the transformer is short circuited through 

 this contact. Of course the resistance of the contact is 

 high as compared to that of the rest of the secondary cir- 

 cuit and it becomes intensely heated. After a few seconds 

 the two pieces fuse together at the joint. When they fuse 

 the contact resistance immediately falls and the joint 

 cools off and is finished. The primary coil of such a trans- 

 former is generally wound for 110-220 volts while the sec- 

 ondary has only one or two turns and so generates only a 

 few volts; the current capacity of the secondary is, how- 

 ever, very high, as perhaps thousands of amperes are re- 

 quired to make a weld quickly. A small transformer 

 designed for making joints in pieces not more than -f^" diam- 

 eter is shown in Fig. 196. The jaws, in which the pieces 

 to be welded are clamped, are seen at the top. 



Instrument Transformers. Instrument transformers are 

 of two types, current and voltage transformers. It is 

 not easy to build an ammeter to carry thousands of 

 amperes, or voltmeters to measure the very high voltages 

 used on transmission lines and so in places where high 

 currents and voltages are to be metered, meters of low 

 ranges are used in connection with suitable instrument 

 transformers. If, for example, the voltage of a 2200-volt 

 line is to be measured, a step-down transformer with a 



