Henry

—The practical unit of self-induction, dseS ra. eee equal to 10° absolute units of induction. It receives its name from J Henry, the eS wo American scientist. The self-induction of TEr a circuit is one henry when the induced E. M. F. is one volt, while the inducing current varies at the rate of one ampere per L TE second. The henry is, therefore, the coefficient by which the time rate of change of the current in the circuit must be multiplied, in order to give the E. M. F of self-induction in the circuit. The for- N a mula for the henry is as follows: flux benrys == — amperes However, as in the case of the farad, the theoretical unit is too great for practical computations, which involves that the millihenry, or 1-1,000th henry, is the accepted unit. In pole-suspended lines the inductance varies as the metallic resistance, the distance between the wires on the cro arm and the number of cycles per second, as mdicated by accepted tables. Thus, for one mile of No 8 B. & S. copper wire, with a resistance of 3.406 ohms, the coefficient of self-induction with 6 inches between centres is.00153, and, with 12 inches,.00175.