the Construction of Galvanometers, ifc. 249 



I venture to hope that these improvements, along with the several adjust- 

 ments and facilities added, will render the instrument more convenient, will 

 increase its sensibility, and contribute to the accuracy of its indications. 



Some persons may conceive that the sensibility, to attain which I have taken 

 so much trouble, is practically redundant. I can only say that I have expe- 

 rienced the absolute necessity of the arrangement described, during my late 

 investigation of the laws of tribothermo-electricity, some of which would have 

 remained unknown but for the excellence of the galvanometer made use of I 

 need not again refer to the delicacy required for thermometrical experiments. 



NOTE. 



I EXTRACT from the Proceedings of the Academy the I'oUowing observations on this com- 

 munication, made by the Rev. Dr. Lloyd, President : 



" The President observed, that all the facts respecting the position of equilibrium of 

 the astatic needle, to which Mr. Donovan had directed the attention of the Academy, and 

 which (as far as he was aware) he has been the first to notice, were immediate consequences 

 of theoretical laws. 



" When two magnetic needles are united by a fixed vertical axis passing through 



theb centres, and perpendicular to both, the moment of the force exerted by the earth 



upon them is the sum of the moments which it exerts upon each needle separately, and is, 



therefore, 



X (M sin u + M' sin u) ; 



in which 31 and M' denote the magnetic moments of the two needles, u and v! the angles 

 which their magnetic axes make with the magnetic meridian, and X the horizontal compo- 

 nent of the earth's magnetic force. In the state of equiUbrium this moment is nothing ; so 

 that if «„ and Mq' denote the corresponding values of u and u, there is 



M smuo + M' svauo =0. (1) 



Consequently, if two lines be taken from any point of the vertical axis, parallel to the 

 magnetic axes of the two needles, and proportional to their magnetic moments, M and M', 

 the diagonal of the parallelogram constructed upon them must lie in the magnetic meridian, 

 when the compound needle is at rest. 



" Again, if we substitute m= «»+ v, u =no' + v, in the general expression of the statical 

 moment, it becomes, in virtue of (1), 



X (M cos ?/o -I- M' cos «o') sin v 



