SPALDING AND SHAW. — COEFFICIENT OF SELF-INDUCTION. 249 



the armature reactions of the alternator, yet a definite current passes 

 through each of the two branches, so that equations (5) and (6) still 

 hold, though Li is no longer constant throughout an alternation but 

 is the effective coefficient of self-induction defined above. 



The practical proof of our conclusion is an easy one. A constant 

 Zi is measured, first by Rayleigh Bridge and then by our heat 

 method. 



The experiment was briefly this. An alternating current was 

 divided between two circuits, both of which had known resistances ; 

 one circuit contained a coil with no iron core whose coefficient of 

 self-induction was to be measured ; the other branch, for simplicity, 

 had no self-induction. The ratio of the currents was determined by 

 the amounts of heat generated in known parts of the two circuits' 

 The connections are shown in Figure 2. 



.4 is a suitable alternator, the one used had a frequency of 200 '^'. 

 a and c are heat coils of German silver wire wound non-inductively ; 

 the resistance of each was 1.288 ohms, and they were placed in oil 

 in order to measure the heat generated, h is the coil whose coefficient 

 of self-induction is to be measured ; it had no iron core. (^ is a resist- 

 ance coil, proportioned to make the currents in the two branches 

 about equal ; it was wound non-inductively and placed in oil to keep 

 it cool. A known standard of variable self-induction might be used 

 and would be less trouble to adjust. The heat developed in the two 

 heat coils, was measured by a thermoelectric junction and a delicate 

 galvanometer ; one end of the junction was kept in melting ice, and 

 the other end measured the heat of each coil in turn. The quantities 

 of oil in the two were equal, so that the ratio of the galvanometer 

 readings gave the ratio of the heat generated in the two. 



We found it desirable to protect our lead-wires from air currents, 

 and to pack the jars containing the heat coils. 



