240 Mr. Michael Donovan 07i certain Improvements in 



It will be found very convenient thus to have a needle susceptible of a 

 strong directive tendency by causing the poles to act in concert, or of gi'eater 

 sensibility by their acting in opposition ; for in this way, and with the occasional 

 aid of the forked magnet, a great range of voltaic forces maybe measured. 

 With ordinary galvanometers, if the sensibility be great, a very feeble deflecting 

 force carries the needle to its maximum, and no greater force can be estimated. 



When the bars have been reversed, as described above, the instrument will 

 be found to possess the usual sensibility of astatic needles ; but not a sufficiency 

 for all purposes, especially when it is intended as a measure of heat. In this 

 case, other proceedings must be resorted to, which, after a few preliminary ob- 

 servations, I will describe. 



During some experiments on different modes of communicating magnetism 

 to needles, results were obtained which I am not aware have been observed by 

 others, or made the subject of inquiry. I sometimes produced a compound needle 

 which found its position directly across the magnetic meridian, instead of coin- 

 ciding with it. As oftenas it was moved into the right position, it would re- 

 turn to the wrong one ; and the more care I took to insure an equally distri- 

 buted magnetism, the more certainly would this perplexing anomaly recur. It 

 Avould be useless to trouble the Academy with all the particulars ; it will suf- 

 fice to say that, in order to investigate the cause of these failures, and provide 

 a remedy, I was obliged to contrive a new instrument. As it would be difficult 

 to refer to its employment without giving it a name, I will here call it a volta- 

 magnetometer, the prefix being sufficient to distinguish it from the magnetometer 

 used for a different purpose. 



The volta-magnetometer consists of a horizontal brass graduated circle, or 

 ring, fixed directly over and parallel to a circular brass plate of the same total 

 diameter. Their distance from each other, maintained by three stout brass 

 studs, is a quarter of an inch, sufficient to allow free space for the oscillation of 

 the compound needle of the galvanometer, which is to be transferred to it as 

 occasion may require, one bar of the needle lying above the graduated ring, 

 and the other between the latter and the circular brass plate. On the circular 

 brass plate is engraved a circle, corresponding with that of the upper graduated 

 ring: both circles are graduated and numbered with such precision, that each 

 degree on the upper circle is exactly vertical to the corresponding one under- 

 neath. The degrees on the lower circle are carried a little farther in towards 



