. The principles of physics. eadings. This instru-ment is of great value to the electrician in dealing with veryweak currents. 467. Tangent galvanometer. — A tangent galvanometer isone so constructed that the current passing through it is pro-portional to the tangent of the angle of deflection produced.To this end it is necessary that the needle be very short (notmore than ?^) in comparison with the diameter of the coil. In its simplest form it consists of a large vertical coil(better two coils, one on each side of the needle, Mg. 387,so placed that the needle isat the center of the commonaxis
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. The principles of physics. eadings. This instru-ment is of great value to the electrician in dealing with veryweak currents. 467. Tangent galvanometer. — A tangent galvanometer isone so constructed that the current passing through it is pro-portional to the tangent of the angle of deflection produced.To this end it is necessary that the needle be very short (notmore than ?^) in comparison with the diameter of the coil. In its simplest form it consists of a large vertical coil(better two coils, one on each side of the needle, Mg. 387, so placed that the needle isat the center of the commonaxis), in the center of whichis either a small compassneedle or a needle suspendedby a silk fiber. A needle thus placed in thefield of a current is acted onby a mechanical couple tend-ing to place it at right anglesto the plane of the coil, and itis deflected until this couple isbalanced by the return coupledue to the earths magnetism.The value of the earths mag- ^^ ^? netic intensity in a horizontal plane is denoted by H.. HrThe formula for the tangent galvanometer is C = r— tan B = 2ir?i K tan 8, in which C is the current strength in C.G.S. units; H, as above, measured in dynes; B, the angle of deflection; and r, the 498 ETHER DYNAMICS. mean radius of the coil of n turns. K, which is called the reductionfactor of the galvanometer, is usually written in the form 2jrn GrIt is made up of two parts, viz. H, which is dependent on locality, and G, which depends on the construction of the instrument, and istherefore called the galvanometer constant. Hence, if the value ofH and G he once found, the strength of any current is calculatedby multiplying the tangent of the deflection angle by the ratio K.As an ampere = lO-i C. G. S. unit, the current strength in amperesis found by multiplying this value by 10. When the scale is divided into degrees, the corresponding tan-gents are foimd by consulting a table of tangents (p. 626). In someinstruments, however, the scale is graduated directly