148 HENRY A. BOWLAXD 



British Association in 1861, who, by their experiments which have ex- 

 tended through eight years, have done so much for the absolute system 

 of electrical measurements. But the actual determination of the unit 

 was made in 1863-4. The method used was that of the revolving coil 

 of Sir William Thomson, the principal advantage of which was its sim- 

 plicity and the fact that the local variation of the earth's magnetism 

 was entirely eliminated and only entered into the calculation as a small 

 correction. The principle of the method is of extreme beauty, seeing 

 that the same earth's magnetism which causes the needle at the centre 

 of the coil to point in the magnetic meridian also causes the current in 

 the revolving coil which deflects the needle from that meridian. When- 

 ever a conducting body moves in a magnetic field, currents are gener- 

 ated in it in such direction that the total resultant action is such that 

 the lines of force are apparently dragged after the body as though they 

 met with resistance in passing through it : and so we may regard Thom- 

 son's method as a means of measuring the amount of this dragging 

 action. 



But, however beautiful and apparently simple the method may appear 

 in theory, yet when we come to the details we find many reasons for 

 not expecting the finest results from it. Nearly all these reasons have 

 been stated by Kohlrausch, and I can do barely more in this direction 

 than review his objections, point out the direction in which each would 

 affect the result, and perhaps in some cases estimate the amount. 



In the first place, as the needle also induced currents in the coil 

 which tended in turn to deflect the needle, the needle must have a very 

 small magnetic moment in order that this term may be small enough 

 to be treated as a correction. For this reason the magnetic needle 

 was a small steel sphere 8 mm. diameter, and not magnetized to satur- 

 ation. It is evident that in a quiescent magnetic field such a magnet 

 would give the direction of the lines of force as accurately as the large 

 magnets of Gauss and Weber, weighing many pounds. But the mag- 

 netic force due to the revolving coil is intermittent and the needle must 

 show as it were the average force, together with the action due to 

 induced magnetization. Whether the magnet shows the average force 

 acting on it or not, depends upon the constancy of the magnetic axis, 

 and there seems to be no reason to suppose that this would change in 

 the slightest, though it would have been better to have made the form 

 of the magnet such that it would have been impossible. The induced 

 magnetism of the sphere would not affect the result, were it not for the 

 time taken in magnetization: on this account the needle is dragged 



