948 Transactions of the American Institute. 



matter of great importance to those purchasing for telegraph or other 

 electrical purposes. 



In short, the capacities of all other instruments combined, for simi- 

 lar purposes, are embraced in this one, in a form so compact and sub- 

 stantial as to be exceedingly convenient, and comparatively safe from 

 injury, by use or from rough handling. 



This apparatus consists of his Tangent Galvanometer and his 

 Rheostat as they have been recently improved. 



2. The tangent galvanometer, of most recent construction, is com- 

 posed of a compass dial, five or six inches in diameter, having a fine 

 steel point in the center, which supports a needle of a form peculiar 

 to this invention. Underneath these are placed coils of several capaci- 

 ties, designed to measure various currents, from those of great inten- 

 sity with but little quantity, to those of great quantity with but little 

 intensity. 



3. The needle is composed of a thin circular plate of tempered steel, 

 in the center of which is fixed an aluminium cup containing an agate 

 to rest upon the point at the center of the compass, or it may be 

 made of three or more oblong plates, riveted upon a flat ring of alu- 

 minium, so trimmed as to form a perfectly circular disk. From the 

 meridian of the disk long slender aluminium pointers extend, to denote 

 the degrees of deflection. The needle being properly polarized, and 

 placed upon the point, obeys every electrical impulse with great 

 celerity. Its weight is scarcely twenty grains, and in some cases not 

 even half that. 



4. The coils are so placed that the current runs parallel with the 

 meridian of the needle. They are half an inch or more wider than 

 the diameter of the disk. By this means all parts of the steel com- 

 posing the needle are subjected to the same inductive influence in all 

 its deflections. 



5. It is a condition indispensable in the construction of a true 

 tangent galvanometer, that the current through the coil should act as 

 uniformly upon the needle in all its deflections as the earth's magnet- 

 ism does ; a narrow coil under a long needle does not fulfill this con- 

 dition ; for, as the extremities of the needle in its deflections pass 

 more and more away from the coil, the inductive influence is less and 

 less, as compared with the earth's influence. 



6. On the contrary, if we place a very broad coil under a long 

 needle, the same difficulty occurs, but in the opposite direction. 

 While the needle is on the meridian it is under the influence of but 

 few convolutions in the middle of the coil, but as it deflects it comes 



