A Modified Deprez-D'Arsonval Galvanometer
C. D. Parkhurst · Transactions of the American Institute of Electrical Engineers · 1893
So far as the writer has observed in all fornas of this galvanometer in actual use, or as illustrated and described in various articles descriptive of its construction and use, and in catalogues advertising it for sale, it has always been arranged for use with a lamp stand and scale for readinig the deflections by the varying position of the spot of light and image of cross-hair reflected from a concave mirror; the protective screen to cuit off air cur- rents has always been of glass, and even thouglh a plain-mirror be substituted for the conicave, there is nothing to pILt the mir- ror in shadow; hence, as the writer knows froim experience, no satisfactory results could be obtained with this form used in connection with a reading telescope.Opinions m-ay difer as to the relative merits of tlhe two systems of reading reflecting galvanoineter deflections.Certain it is, owever, that the lamp stand and scale form a clumsy and bulky outfit.To get a good, sharp and.clear imiage of the spot of light and its cross-hair needs a very strong light or a darl closet in which to use the galvanometer.Independent of its bulkiness and want of portability, it is a hot and inconveniient mnethod of reading, and one that, in the writeiK's opinion, bears no comparison with the ease and refinelents of reading, obtain- able with a reading telescope worthy of the name.Since to show up well in a reading telescope the nirror mnust be in a heavy shadow, and the scale upon the telescope be well illuminated, the writer found that a modification of the ordinary D'Arsonval galvanomneter in Ilis possession was inecessary, and 1893.]PARKII URST ON GAL VANOMETERS.27tthis niodification will Inow be described, reference being made to the drawings herewith drawn to scale one-quarter size.The horse-shoe or rather U-shaped magnet was dropped down comnpletely through the base, so as to project below its lower sur- face sufficienitly to allow the lower tension spirig to be mounted on the inder side of the base, where it could be reached at any ti'me to adjust the tensioni of the suspension.This became nec- essary owing to the fixed brass case which enclosed the instrument, as showrn in the illustration, this fixed brass case being the