Magnetic Induction in Straight Iron Rods. 481 



means of a standard solenoid S (also used for magnetizing the 

 specimen) and secondary coil A. This coil could be placed 

 inside the solenoid coaxial with it, and its dimensions were 

 accurately known. It was always in the galvanometer circuit 

 and served to calibrate the galvanometer absolutely. This 

 was done at the conclusion of taking each set of observations 

 by removing the specimen from the solenoid and inserting 

 the secondary coil. The advantage of this method of calibra- 

 tion is that the conditions of the calibration are the actual 

 conditions of the experiment. 



Although it is generally assumed that the quantity of 

 electricity discharged through a suspended coil ballistic 

 galvanometer is proportional to the deflexion, it was found 

 that for this particular galvanometer the correction was about 

 2 per cent, positive for small deflexions, and 2ijr negative for 

 large deflexions. 



This was no doubt due to the effect of damping, as there 

 was a large air-vane fixed to the upper part of the coil and 

 partially enclosed. 



The magnetizing current was measured by observing the 

 fall of potential on a standard resistance K. This was done 

 with a Weston direct-reading voltmeter which had been 

 previously calibrated by a potentiometer and Clark cell in the 

 usual way. 



The solenoid employed was 56 centim. long, 4*03 centim. 

 mean diameter, and had 17*82 turns per centim. 



The specimen was kept central in the solenoid by means of 

 an ebonite tube D, whose inside diameter was just large 

 enough to allow the specimen to pass inside^ and the outside 

 diameter large enough to fit the solenoid tube closely. The 

 larger part of the specimen was allowed to project from this 

 tube so that the search-coil could be placed in any position 

 from the centre to the end. To the search-coil was attached 

 a small brass rod P, which projected beyond the end of the 

 solenoid. This rod served to move the search-coil along the 

 specimen, the outer end indicating on a suitably placed scale 

 the position of the coil. 



The search-coil could ihus be placed in any desired position. 



Prof. Fleming in a paper on "A Method of Measuring 

 Hysteresis Loss in Straight Iron Strips " (Phil. Mag. Sept. 

 1897 ; Elec. Feb. 5-March 11, 1897), gives the results of a 

 series of experiments to determine the distribution of induction 

 in iron rods or bundles of strips cut from transformer plates. 

 The object of the experiments was to determine the value 

 of B . His method of measurement was to place the 



