98 HENEY A. EOWLAND 



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 galvanometer-needle when the bar is demagnetized. It does not 

 seem to have been noticed that what we ordinarily consider the mag- 

 netic 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 proportional to the surface-integral of magnetic induction 

 along a given portion of the Itar. In other words, the quantity measured 



is Q instead of -^L. 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 meas- 

 ures Q f , 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 per- 

 pendicular 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 most 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 positions 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 tem- 

 porary one) between the magnet and current; so that the distribution 

 remains unchanged. Hence it seems to me that this method is the 

 only one capable of giving exact results directly. 



