Messrs. Ayrton and Mather on Galvanometers. 445 



Prof. Holman admits the accuracy of our reasoning that 

 there is a certain space in which the wire should either be 

 left out, or be oppositely connected, but he considers that we 

 overestimated the volume of this space because we neglected 

 " the fact that the field over the remainder of the needle is 

 not the same as at the poles, either in direction or strength." 



And to support his argument he proceeds to consider what 

 ought to be the boundary of the space in the ciise of " a very 

 thin uniformly magnetized prismatic needle " (the italics are 

 ours) without observing that this is exactly the case we dealt 

 with, and, therefore, must lead to exactly the conclusion we 

 arrived at. For no part of such a longitudinally magnetized 

 needle, other than its ends, contributes to the deflecting 

 moment when placed in any weak magnetic field, since no 

 free magnetism exists except at its ends. Hence his objection 

 that the field is not uniform throughout the length of the 

 needle has no weight whatever in the very example he has 

 himself selected. 



We may also call attention to another error into w T hich 

 Prof. Holman has fallen. He gives as the value of the 

 deflecting moment, produced by a current in a coil on a very 

 thin uniformly magnetized prismatic needle, the expression 



2 J m ./cos 6 . ds, 



" m being the strength of pole of any thin transverse section 

 or shell of the needle, ds the thickness of that section,/ the 

 field-intensity at that point, and 6 the field-direction angle 

 with the axis of the coil.'' 7 But this expression could only be 

 correct if every part of the needle were equidistant from the 

 axis of rotation. And even if allowance were to be made for 

 this not being the case by introducing s, the distance of a 

 section from the axis, the expression 



1 

 m ./cos 6 . sds 







would still only give the correct value for the deflecting 

 moments in the case of " a very thin uniformly magnetized 

 prismatic needle " by making m equal to nought for all points, 

 except at the ends. And when that is done, the conclusions 

 arrived at by Prof. Holman are profoundly modified. 



As to the confirmation by experiment of his conclusion that 

 the deflecting moment of a coil of diameter about half the 

 length of the needle is nil when the coil is placed close to the 

 needle, that merely proves, we think, that the needle he used 

 in his experiments was not uniformly magnetized. It is, of 

 course, well known that it is almost impossible to obtain 



•J"; 



