the Construction of Galvanometers, i^c. 237 



may be replaced. Torsion, even of so slender a fibre, lias more effect in im- 

 pairing sensibility tlian might be supposed. 



The frame of the coil is screwed on a strong circular plate of brass, sur- 

 rounded by a hoop sufficient to secure the French shade which covers the 

 whole. The solid vertical axis of this plate turns smoothly and slowly in 

 a ground socket, by means of a crown wheel fixed to the plate, acted on 

 by a pinion and thumb-screw attached to the tripod on which the whole in- 

 strument stands. The levelling screws pass through the tripod, and rest in so 

 many holes in a circle of brass screwed to a mahogany stand, in the bottom of 

 which should be a drawer to contain various necessaries. On the stand fits a ma- 

 hogany cover or box, with fastenings. The ends of the coil pass down through 

 two holes in the circular brass plate, bushed with ivory, and are soldered under- 

 neath the plate, each to an insulated binding screw, situated one on each side of 

 the meridian line. Between the binding screws, and affixed to the circular brass 

 plate, is a horizontal, hollow, square trunk, three inches in length : each side of 

 the interior measures about three-sixteenths of an inch : it is fixed in the di- 

 rection of the magnetic meridian line of the galvanometer, and has a continu- 

 ation of this line engraved on its upper surface. At the end of the trunk, and 

 on the meridian line, is erected a short, sharp brass point, on which may be oc- 

 casionally placed a common compass needle. When the galvanometer is to be 

 used, the compass needle is to be placed on the brass point, and the thumb- 

 screw underneath turned until the point of the compass needle and the meridian 

 line coincide precisely. The instrument is now set; the compass needle is to 

 be removed, lest it interfere with the astatic needle: the latter should also have 

 been previously removed, as it would equally interfere with the compass needle. 

 The necessity of thus setting the instrument by a detached compass needle, 

 and of not depending for this service on the astatic needle, although the latter 

 is usually relied on, will abundantly appear hereafter. 



The trunk is made hollow, because it has other duties to perform. It is 

 intended to contain one prong of a magnet made exactly in the shape of what 

 musicians call a " tuning fork," except that the prongs are nearer together, 

 beinar iust the distance that the two needles are from each other on their 

 spindle. This magnet slides in and out of the trunk, which it exactly fits ; it 

 is so placed, that one prong is always vertical over the other when it is in use. 



2 i2 



