5-A1 STUDY AND OPERATION. *33 



which would give rise to excessive current and burn out the 

 transformer. 



8. Marking Polarity. It is convenient and common to desig- 

 nate the polarity of several coils by some systematic marking; 

 thus, all terminals of one polarity may be marked prime (') and 

 those of opposite polarity be unmarked. To connect coils in 

 parallel, marked terminals are connected to one line and un- 

 marked terminals to the other; to connect in series, the marked 

 terminal of one coil is connected to the unmarked terminal of 

 the next coil, as in Figs. 2, 3 and 5. 



In marking polarity it is always best to have the marked sec- 

 ondary terminals of the same polarity as the marked primary 

 terminals. Some positive convention* of this kind becomes im- 

 portant whenever proper polarity is essential, as in the case of 

 transformers supplying the same secondary main, transformers 

 on polyphase circuits (Exps. 6-A and 7~A), transformers used 

 for 4 reducing the current or voltage supplied to wattmeters, etc. 



For polarity and ratio tests on commercial transformers, see" 

 Appendix I. 



9. Ratio Test.f Compute and verify, experimentally, the 

 different ratios of voltage transformation, E^-^-E^ which are 

 possible with the transformer. At any instant the electromotive 

 force of a coil is, by Faraday's Law, 



where S is number of turns and <f> is flux. The instantaneous 

 value of the voltage, and hence the effective or virtual value, 

 is accordingly proportional to the number of turns, and the 

 ratio of voltages in any two coils is the ratio of the number of 

 turns in the coils. (See Appendix I.) 



*In connecting together transformers of different makes, care must be 

 taken, for their polarities may be indicated by different systems. 



t For current ratio and tests on commercial transformers, see Appendix I. 



