*4 2 TRANSFORMERS. [Exp. 



ratio of the cross section of copper to the total cross section of 

 the windings, i. e., to the cross section of copper plus insulation 

 and air space. Similarly the space factor for the iron is the 

 ratio of its net to gross section. 



APPENDIX I. 



POLARITY AND RATIO OF COMMERCIAL TRANSFORMER. 



28. Polarity; Alternating Current Method. The coils are con- 

 nected in series, two at a time, and notice is taken whether the 

 voltage around the two is the sum or the difference of the separate 

 voltages. There are several ways in which this can be carried out. 



As an example, let us take a transformer with two primaries for 

 1,000 volts each and two secondaries for 50 volts each. Connect the 

 _^_______ L two i,ooo-volt primaries in series and con- 



so Volt A | 5 , Q j nect the terminals of one* of the primaries 



^L ^2 je-ioo to a l w potential supply circuit, say 50 



1 sb | volts, as in Fig. 5. If a voltmeter across 



B * the two coils together reads zero, reverse 



FIG. 5. Polarity test the connections of one of the coils. The 



by alternating current yoltmeter should then fead IQQ vohs acrQSS 



method. 



the two coils together, and 50 volts across 



each one separately. Terminals A and B are now of one polarity; 

 terminals A' and B' are of the opposite polarity, to be marked with 

 a prime (') or X- 



Each secondary is then connected in series with one primary, the 

 primary being connected to the 5o-volt supply circuit; the secondary 

 in series with it is so connected that the voltmeter reading around the 

 two coils in series is greater (52.5 volts) than the potential from the 

 mains (50 volts). If the reading is less (47.5 volts), reverse the 

 secondary. Secondary terminals are marked with a prime (') or X 

 to correspond with the primary. 



Small transformers are commonly so wound that, when the primary 

 and secondary leads on one side of the transformer are connected 



* If the two coils in series were connected to the supply circuit, a burn- 

 out might result if the coils were opposed to each other. 



