Properties of the Alloys of Cast-iron and Aluminium. 605 



This rocker enabled the battery B to be connected at will 

 to either the primary of the ring R or the primary of the 

 standard mutual inductance-coil M. 





Fia:. 1. 



e: h 



The rocker D was directly connected to a variable resistance 

 C and a reversing-key K. 



To K was connected a battery of five storage-cells B and a 

 Weston's voltmeter V, to the terminals of which was attached 

 a resistance r t . 



The value of this resistance was one ohm, and hence the 

 reading of the voltmeter in volts was numerically equal to the 

 primary current in amperes. 



This resistance w T as composed of thick platinoid wire twisted 

 spirally and suspended by threads in a box with perforated 

 sides and cover to allow the free circulation of air round the 

 wire. 



It was found by trial that the resistance of this coil was 

 not appreciably changed by the passage of the greatest current 

 used in the experiments. 



The adjustable resistance C consisted of a number of 

 coils of platinoid wire attached to a brass frame. The free 

 ends of these coils were connected to a series of stops on the 

 upper circle. Contact was made by means of a T-shaped brass 

 arm revolving round the axis of the frame, and of such dimen- 

 sions as to enable the resistance to be varied without breaking 

 the circuit. This arrangement of coils was found very con- 

 venient for varying the current ' by steps/ 



The Secondary Circuit. 



The secondary of the ring and the secondary of the mutual 

 inductance-coil were connected permanently to one another 



