I3 2 TRANSFORMERS. [Exp. 



following outline is written specifically for such a transformer 

 having four equal coils, each for 55 volts, but the experiment 

 may be modified so as to apply to a transformer wound in some 

 other manner. See Appendix I. for polarity and ratio tests on 

 a commercial transformer. 



PART II. TESTS. 



6 Polarity Test ; Series and Parallel Connection of Coils. 

 Use one coil as a primary and connect it (with a resistance in 

 series as a safeguard) to a 55-volt alternating current supply. 

 Use the other three coils as a secondary, connecting them in series 

 in such a manner that the three electromotive forces are additive 

 and do not oppose one another. To prove this, measure the 

 electromotive force of each coil and the electromotive force 

 across the three; the latter value should equal the sum of the 

 three other values. This establishes the polarity of the coils. 



7 With the primary circuit unchanged, join the three sec- 

 ondary coils in parallel. In doing this it is necessary to make 

 sure that terminals about to be connected together are of like 

 polarity, as in connecting batteries. When the polarity has been 

 already established, as in the preceding section, the proper parallel 

 connection can be readily made. The following procedure, how- 

 ever, insures the right connection independent of previous knowl- 

 edge of polarity, two coils being first joined 'in parallel and the 

 third coil being then joined in parallel with these two. To deter- 

 mine which terminals should be connected together to connect 

 two coils in parallel, join a terminal of one coil to a terminal 

 of the other coil and connect a voltmeter to the two remaining 

 terminals. If the voltmeter reads zero, the two terminals con- 

 nected to the voltmeter may be joined together and the two coils 

 will be in parallel. If the voltmeter does not read zero or very 

 nearly zero, the terminals connected to the voltmeter cannot be 

 joined together without causing a short circuit of the two coils 



