[Illustration: Fig. 520.--Graduation of tangent galvanometer scale with divisions representing tangent values. In the figure let a tangent OT be drawn to the circle, and along this line let any number of equal divisions be set off, beginning at O. From these points draw lines back to the center. The circle will thus be divided into a number of spaces, of which those near O are nearly equal, but which get smaller and smaller as they recede from O. These unequal spaces correspond to equal increments of the tangent. If the scale were divided thus, the readings would be proportional to the tangents.] Ques. Upon what does the sensibility of a tangent galvanometer depend? Ans. It is directly proportional to the number of turns of the coil and inversely proportional to the diameter of the coil. Ques. How may the tangent galvanometer be used as an ammeter? Ans. The strength of the current may be calculated in amperes by the formula given below when the dimensions of the instrument are known. The needle is supposed to be subject to only the earth's magnetism and to move in a horizontal plane. The current is calculated as follows: (1) amperes = ((H × _r_)/N) tan _x_ in which H = constant from table below; r = radius of coil; N = number of turns of coil; x = angle of deflection of needle. The constant H, given in the following table represents the horizontal force of the earth's magnetism for the place where the galvanometer is used. Each value has been multiplied by (2[pi])/10 so that the formula (1) for amperes is correct as given. Table of Galvanometer Constants.--Values of H. Boston | .699| Chicago | .759| Denver | .919| Jacksonville | 1.094| London | .745| Minneapolis | .681| New York | .744| New Haven | .731| Philadelphia | .783| Portland, Me. | .674| San Francisco | 1.021| St. Louis | .871| Washington | .810|