296 Mr. F. Guthrie on an Absolute Galvanometer. 



This is the general solution; and where the limits are infinity 

 and cipher, there results 



(l-E)-'.|.{l-e-} ff0 =|.{H-E»}-V 



which is the same as given by Mr. Glaisher. 



It is evident that there are numerous theorems of the same 

 kind, such as 



cos E# . a Q =a — ~-^ + &c, 



a oc 

 sin Ea?.ff sra^*- ^p^rg* &c *' 



which will give definite results between the limits infinity and 

 cipher. 



Yours obediently, 



James O'Kinealy, B.C.S. 



XLII. On an Absolute Galvanometer. 

 By Frederick Guthrie *. 



MESSRS. ELLIOTT have constructed for me a galvano- 

 meter which will, I believe, be found to possess for some 

 purposes certain advantages over those at present in use. Its 

 principle depends upon the measurement of the current-strength 

 by the measurement of the mechanical force necessary to bring 

 1 a given distance of one another two electromagnets, which 

 are excited by the current in such a fashion that they repel one 

 another. 



The current enters at a by the screw-clamp ; thence it passes 

 beneath the circular wooden stand C along the copper wire a b. 

 It rises vertically and coils round a soft iron mass/, which lies 

 horizontal and tangential to the axis of the instrument. It 

 passes down and across the centre of the board, then rises and 

 coils round a soft iron mass /', exactly similar and similarly 

 placed to f 9 but on the opposite side of the instrument. Having 

 encircled/', the current-bearing wire again descends, and carries 

 a mercury- cup g, through whose bottom it passes, and which is 

 exactly in the axis of the instrument. The current then leaves 

 the mercury by the wire z", which dips into it. It then traverses 

 the wire around the iron, m. Thence it crosses the instrument 

 and forms a spiral around m\ after which it passes into the mer- 

 cury-cup h, and so to the binding-screw c. The spirals are such 



* Read before the Physical Society, May 23, 1874. Communicated by 

 the Society. 



