4-A] SERIES AND PARALLEL CIRCUITS. 1 1 1 



24. Vary the adjusting resistance,* and in this way take sev- 

 eral sets of readings. 



If there is any question as to the accuracy of the instruments, 

 assume the ammeter and voltmeter to be correct and determine a 

 correction for the wattmeter, so that in (a) the watts as read by 

 the wattmeter are equal to the product of volts and amperes, as 

 read by the voltmeter and ammeter. This serves as a calibration 

 of the wattmeter, to be used in this and subsequent tests. 



25. Take readings, in a like manner, at a second frequency. 



26. (&) Coil Alone. f Repeat (a) using the coil R 2 L 2 alone, 

 as in Fig. 6, instead of the resistance R t . 



27. Take readings at a second frequency. 



28. Effect of Iron. Gradually introduce an iron core and 

 watch the ammeter ; or, introduce iron wires, a few at a time, thus 

 gradually increasing the amount of iron. At present, only the 

 general effect of iron is to be noted and explained; a more com- 

 plete study of iron in the form of a closed magnetic circuit is 

 made in the subsequent experiments on the transformer. 



29. (c) Resistance and Coil in Series. Connect the resist- 

 ance R : and the coil R 2 L 2 in series, and, together with an adjust- 

 ing resistance, connect to the supply, as in Fig. 8. For a certain 

 current, take readings of the voltage drop, the current and the 

 watts consumed as follows: first, for the resistance; second, for 



* (243). This adjustment should be so made that the readings of the 

 various instruments are taken at open parts of the scales. 



t(26a). Series and Parallel Connections. It is instructive to use a 

 coil with two equal windings. In this case, the regular tests should be 

 made with the two windings either in parallel or in series and additive, 

 i. e., setting up magnetic flux in the same direction. If one winding is 

 reversed, it will oppose the other so that the resultant flux (and hence 

 the impedance) is small. A few volts may cause a very large current. 



Preliminary Test. With the resistance Ri in series as a safeguard, to 

 avoid excessive current, measure the current and voltage and determine 

 the impedance of each winding alone and of the two windings connected 

 in series and in parallel, additively and differentially. The additive wind- 

 ing is inductive; the differential winding is non-inductive, except so far 

 as there is magnetic leakage. 



