266 Mr. H. A. Rowland on Magnetic Distribution. 



tribution, as, for instance, when the current around an electro- 

 magnet varies. 



The fifth method is that used lately in some experiments 

 of Mr. Sears (American Journal of Science, July 1874), but 

 only adapted to temporary magnetization. At a given point 

 on the bar a small coil of wire is placed, and the current 

 induced in it measured by the swing of the galvanome- 

 ter-needle when the bar is demagnetized. It does not seem 

 to have been noticed that what we ordinarily consider the 

 magnetic distribution is not directly measured in this way ; and 

 indeed, to get correct results, the magnetization should have 

 been reversed, seeing that a large portion of the magnetization 

 will not disappear, on taking away the magnetizing-force, where 

 the bar is long. The quantity which is directly measured is 

 the surface-integral of the temporary magnetic induction across 

 the section of the bar, while the magnetic surface-density is pro- 

 portional to the surface-integral of magnetic induction along a 

 given portion of the bar. In other words, the quantity measured 



is Q instead of —^-. We can, 'however, derive one from the 



other very easily. 



The sixth and last method is that which I used first in 1870, 

 and by which most of my experiments have been performed. 

 This consists in sliding a small coil of wire, which just fits the 

 bar and is also very narrow, along the bar inch by inch, and 

 noting the induced current over each inch by the deflection of 

 a galvanometer-needle. This measures Q e , except for some 

 corrections which I now wish to note. In the first case, to give 

 exact results, the lines of force should pass out perpendicular to 

 the bar, or the coil must be very small. But even when the 

 last condition is fulfilled errors will be introduced at certain por- 

 tions of the bar. The error is vanishingly small in nrost cases, 

 except near the ends ; and even there it is not large, except in 

 special cases; for at this part the lines of force pass forward 

 toward the end of the bar, and so the observation next to the 

 end may be too small, while that at the end is too large. The 

 correction can be made by finding where the lines of force 

 through the centre of the section of the coil in its two posi- 

 tions meet the bar. The error from this source is not large, 

 and may be avoided to a great extent. 



One very great advantage in the method of induced currents 

 is the facility with which the results can be reduced to absolute 

 measure by including an earth-inductor in the circuit as I have 

 before described (Phil. Mag. August 1873). There is also no 

 reaction (except a temporary one) between the magnet and cur- 

 rent ; so that the distribution remains unchanged. Hence it 



