i 



104 Notice of a Galvanometer. 



steel wire and is partly neutralized by being connected with a second 

 and more delicate needle, made of the smallest species of main 

 spring. This second needle is parallel to the principal one, three 

 eights of an inch above it, and with its poles reversed in the manner 

 of M. Nobili. It is intended to act chiefly as an index and swings 

 above the card. The two needles are firmly connected together by 

 a brass wire. The whole is closed by a glass cover in the manner of 

 a common compass, which it nearly resembles in external appearance. 

 The glass is however pierced in its center by a hole half an inch in di- 

 ameter, for the insertion of a perpendicular brass tube, four inches 



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high. This tube includes the suspension filament which is fastened 

 to the center of the cap the latter being closed at the top. The pro- 

 jecting wires ascending in an arch, and approaching within a quarter 

 of an inch, terminate, the one in a five cent piece of silver, and the 

 other in a corresponding disc of zinc. Three projecting brass feet, 

 with each a levelling screw, support the instrument. 



A wine glass of water, applied once to the discs, will deflect the nee- 

 dle from two to five degrees ; by repeated applications the needle 

 may be made to vibrate through sixty degrees. When the conjunc- 

 tive wires of a calorimotor of one and half square feet of zinc, 

 filled with rain water, are laid on the poles of this galvanometer, the 

 needle is deflected to ninety degrees which is the maximum. If it 

 be suffered to remain thus in action, it will retain the needle at about 

 seventy degrees for twenty four hours ; how much longer I have not 

 yet determined. 



The advantages which it seems to me, my instrument possesses 

 are, that the " multiplying" coil being brought the nearest possible 

 to the needle must affect it the most sensibly ; being entirely enclosed, 

 it is not disturbed by the agitation of the air ; it is compact and 

 elegant in its form. 



Fig. 1. Is a plan of the ring R, and coil of wire W, half size. 



Fig. 2. Is a section of the whole instrument. B. The box. R. 

 The ring. W. The wire. N. S. The needle, n. s. The index. 

 D. The dial cut out in the center to show the wires and to permit 

 the needle to be introduced or withdrawn readily, through the open- 

 ing O. C. The wire connecting the index and needle. G. The 

 lass cover. T. The brass tube including the suspension filament F. 



Fig. 3. The perspective view, in which the corresponding parts 

 are marked with the same letters as in fig. 2. P. Is an ivory pin 



