346 Mr. G. J. Stoncy on the Correction for the 



After quoting this formula, the Professor proceeds to deduce 

 one which he proposes to substitute for it; but as his funda- 

 mental assumption, p'=p • /($), is at variance with the familiar 

 fact that the action of the element of a current on a magnet de- 

 pends on its inclination as well as its distance^ it is not necessary 

 to scrutinize another hypothesis inconsistent with that just stated, 

 which he afterwards introduces*. 



The formula thus obtained through a disregard of the laws 

 ascertained by Ampere, is supported by experiments which 

 appear equally independent of those due to Ohm. The internal 

 resistance of an electromotor was varied by immersing the plates 

 successively to different depths, and observations are recorded 

 with the assumption that the intensity of the current changed in 

 the same ratio, although there does not seem to have been any 

 alteration made of the external resistance. I need not, then, con- 

 test other parts of the experiment, although I believe it would be 

 difficult to render the method by which the change of internal 

 resistance was estimated practically trustworthy; nor need I 

 dwell on the peculiarity of testing the established formula, ex- 

 piressly limited to cases in which the needle is sufficienthj short to 

 warrant our neglecting the fourth power of\, by making experi- 

 ments with a galvanometer the needle of which had, to use 

 Prof. Zenger's description of it, an " enormous length/'' 



Under all these circumstances, it can scarcely be matter of sur- 

 prise that no accordance was found between the observations and 

 the established formula : and we seem compelled to regard the 

 moderate agreement which was obtained between an erroneous 

 formula and faulty experiments as a coincidence without scien- 

 tific import. 



Although the topic is quite unconnected with Prof. Zenger's 

 remarks, whom the error seems to have escaped, I may be 

 allowed to avail myself of this opportunity to correct a mistake 

 in my paper, to which Prof. Curtis of Queen's College, Gal- 

 way, was so good as to direct my attention some months ago. 

 Having examined the corrections occasioned by a derangement 

 from its intended position of the magnetic centre of the needle, 

 regarded as the point round which the needle rotates, I endea- 

 voured f, by a merely geometrical process, to include the error 

 which a separation between the point of suspension and the 

 magnetic centre would introduce. This was done under the mis- 



* Professor Zenger's equation, 



S:S' = AN' 2 :AN2 



(see p. 530), is mathematically inconsistent with his former hypothesis. It 

 is equally inconsistent with Ampere's law. 

 t Phil. Mag. February 1858, p. 138. 



