﻿of finding the Forces acting in Magnetic Circuits. 93 



<The bars were kept straight and aligned by well-fitting 

 glass, or brass, or fibre tubes at the plane of contact. The 

 fit was always just so good that no correction for friction was 

 necessary. The magnetizing current was measured by a 

 Siemens dynamometer, which was compared with suitable 

 members of a chain of Kelvin balances. It was found that 

 in this instrument the readings were correct within the limits 

 of accuracy of reading. The current was supplied by storage- 

 cells. 



The induction-coils were wound on brass bobbins with 

 proper precautions. The bobbins were of different diameters, 

 and were compared ballistically, and found to give identical 

 results ; hence it was concluded that they were all free from 

 leakage errors. 



The ballistic galvanometer was a fine instrument, specially 

 made for this kind of work. It was calibrated by turning 

 over a large coil which was splendidly wound, and which has 

 been checked in many ways. I made use of the values for 

 the vertical force obtained about nine months ago by Mr. Farr, 

 from a long series of experiments in my laboratory under the 

 best and most careful conditions, and with the Kew apparatus. 

 In all cases the induction- throw on the galvanometer was 

 checked by reversal of the galvanometer-corrections, except 

 when experiment showed that nothing was gained by such 

 reversals, the instrument being in another part of the labora- 

 tory to the magnetic system. In all cases large resistances 

 from a box of coils, constructed and calibrated by myself 

 from Cambridge standards, were inserted in the circuit so as 

 to give the best range for the galvanometer. 



§ 8. Results of Experimenting. — The general result was that 

 I got rather worse agreement than was noted by Bosanquet, 

 especially at low inductions. I therefore set myself to find 

 out the reason of this. I am ashamed to say how T long it took 

 me to clear up the difficulty. I investigated the following 

 possible causes. 



(1) Imperfection of galvanometer law. This w r as got over 

 by adjusting resistances till the same deflexion was obtained 

 both on turning over the earth-inductor and on magnetizing 

 the iron. 



(2) Effects due to residual state of the iron. These were 

 got rid of by demagnetizing the iron by an alternator and slide- 

 resistance, and observing magnetizations with the current in 

 both directions, and also on reversal. By comparing the 

 three sets of deflexions I assured myself that the discrepancv 

 was not due to any error of this kind. A similar procedure 

 when taking the tractions led to a like result. 



