233 



X. — On certain Improvements in the Construction of Galvanometers : on Galva- 

 nometers in general : and on a new Instrument for measuring the relative Force 

 of Magnetism in compound Np.tdles intended to he nearly Astatic. By Michael 

 Donovan, Esq. 



Read May 22, 1848. 



L HE galvanometeu, in the present day, lias become a most important instru- 

 ment of research, whether it be considered as a measure of electricity or of heat. 

 In the latter capacity, it exceeds all others in sensibility and the promptness of 

 its indications ; and when it is recollected that by its aid facts have been ascer- 

 tained which had been erroneously represented by the thermometer, that de- 

 grees of heat have been estimated to which the thermometer was in some cases 

 almost insensible, and in others inapplicable, its value and capabilities need no 

 encomium. But it is necessary that, for delicate purposes, we should have the 

 instrument in its utmost attainable state of perfection. Under such impres- 

 sions, I have made some efforts to improve it ; and beheve I have succeeded 

 in rendering it more certain and accurate in its results, as well as more sensible 

 in its indications. 



The sensibility of a galvanometer is increased when its construction is such, 

 that the two layers of the Avire coil, between which the compound or double 

 needle lies, are as near as possible to each other, the consequence being that the 

 lower bar of the compound needle will be very close to both. The upper bar 

 should be equally near the upper layer of the coil. The proximity of the bars 

 to the coil is one of the great sources of sensibility; but, to permit this, many 

 things must be attended to. 



In the first place, however carefully the wire of the coil may have been 

 covered with silk or cotton, there will always be a number of fibres projecting 



