Magnetic Induction in a Long Iron Bar. 



263 



coil was at the centre of the bar, and at other definite positions 

 up to the end. For the determination of curve A it was kept 

 at the centre. The larger magnetizing-currents were read on 

 Weston standard ammeters calibrated by comparison with a 

 Crompton potentiometer; the small currents and those on 

 the steep parts of the magnetization-curves were read directly 

 by means of the potentiometer. The currents were adjusted 

 by ordinary resistances for the larger values, or by a simple 

 circular potential-slide as shown developed in fig. 1 for the 

 smaller ones. 



Fie-. 1. 



The ballistic galvanometer used was a Crompton " Midget " 

 D'Arsonval, and was tested for proportionality by means of a 

 standard field of known mutual-induction coefficient ; it was 

 found to be practically quite accurate. The standard field 

 was also used throughout the experiment to calibrate the 

 ballistic galvanometer, the connexions being shown in fig. 1. 

 Fig. 2 shows the arrangement of the search-coil and specimen 

 rod. 



Fig. 2. 



The results on reduction were carefully plotted to a large 

 scale, and fig. 5 is a reduced copy of this curve ; it connects 



