360 Improved Galvanometer. 



the needles of which are each about eighteen inches in length. The 

 instrument is furnished above with a circle graduated into three hun- 

 dred and sixty degrees* Agreeably to the usual construction, the 

 needle being within, the coil is subjected both above and below to 

 the concurring influence of a current passed through the coil. In 

 this predicament is the lower needle in the adjoining figure. When 

 a needle is situated outside of the coil like the upper one in this fig- 

 ure, the influence of the lower portion of the coil, so far as it ope- 

 rates, must counteract that of the upper one. Yet when the lower 

 portion of the metallic coil is at a distance from the upper portion of 

 about one third of the length of the needle, and this is situated 

 very near to the upper portion as here represented, the influ- 

 ence of the latter may so far predominate as to render the indica- 

 tions very nice ; while they are as much more easily seen and esti- 

 mated by means of the graduated circle, when, as in the situation of 

 the upper needle, nothing intervenes between it and the eye. 



In another instrument of the same dimensions I have used only a 

 semicircle for the graduations, which, excepting the appearance, an- 

 swers as well. 



In lieu of wire, a coil of tin foil of about an inch in breadth and 

 eighty feet in length, separated by thin paper, may be used, but a 

 copper wire of No. 16, and of about one hundred and eighty feet 

 in length, coated with shell lac varnish, will be more efficacious. 



The coil of tin foil or varnished copper wire, is wound about the 

 paralellogram C C C C. The ends of the coil are severally solder- 

 ed, or screwed, under the basis of the gallows screws S S. 



When both needles are placed upon the pivot at the same time by 

 the repulsion of their similar poles, they will diverge from the meri- 

 dian unless they be in a reversed situation, in which case they will 

 appear as in the engraving, the north pole of one pointing north, the 

 north pole of the other needle south. When, under these circum- 

 stances, a discharge is made through the surrounding coils, the con- 

 sequent movements are very striking. 



The clean surfaces of disks of zinc and copper, each an inch in 

 diameter, separated by paper moistened with pure water, are suffi- 

 cient to move the needles sensibly. The wires W W are used for 

 the purpose. They are attached to the instrument by gallows screws 

 SS. 



The level of the machine is preserved by the aid of four screws, 

 of which only three can be seen in the drawing at T T T. 





