138 



THE TRANSFORMER. 



two resistances. The resistances of the windings must be 

 small compared with the resistances in series with the cell. 



EXPERIMENT XXIV. DETERMINATION OF RATIO OF 

 WINDINGS OF A TRANSFORMER (WITHOUT ALTERNATING 

 CURRENT). 



PIAGRAM OF CONNECTIONS. 



Ri 



FIG. 64. 



I., II. 

 B lt R 2 



R 

 



8 



Windings whose ratio is to be determined. 

 Adjustable known resistances (e,g., the arms of a 



P.O. Wheatstone's bridge). 

 Battery of a few cells. 

 Galvanometer or milli-voltmeter. 

 Switch. 



Instructions. Connect the two windings to be compared 

 in parallel to a battery consisting of two or three cells in 

 series with a key-switch, inserting in series with each winding 

 a variable known resistance, e.g., a plug resistance box. Care 

 must be taken that the windings are connected in such a way 

 that the field due to the currents in them are oppositely 

 directed. Connect a third winding on the same magnetic 

 circuit to a galvanometer or milli-voltmeter. Adjust the 

 resistances in series with the coils until no throw of the 

 galvanometer is produced on breaking the battery circuit. 

 When this has been done, the ratio of the windings will be 

 the same as the ratio of the resistances in series with them. 

 The resistances employed should be large enough to make 

 the resistance of the transformer windings negligible in 

 comparison with them. If this is not the case, the resistance 

 of the windings must be included in the calculation, the 

 ratio of the number of turns in the windings being equal to 

 the ratio of the total resistances of the two branch circuits. 

 The principle of the method is as follows : 

 The magnetising effect of each winding is proportional 

 to the number of turns x current sent through it by the 

 battery. 



