It is this last method of transmission which remains to be discussed. The author, however, feels himself obliged to restrict himself in this matter to a mere summary; and, indeed, it is English physicists and engineers who have taken the technology of electricity out of the region of empiricism and have placed it on a scientific and rational basis. Moreover, they are also taking the lead in the progress which is being accomplished in this branch of knowledge, and are best qualified to determine its true bearings. When an electric current, with an intensity, i, is produced, either by chemical or mechanical work, in a circuit having a total resistance, R, a quantity of heat is developed in the circuit, and this heat is the exact equivalent of the force expended, so long as the current is not made use of for doing any external work. The expression for this quantity of heat, per unit of time, is Ai²R; A being the thermal equivalent of the unit of power corresponding to the units of current and resistance, in which i and R are respectively expressed. The product, i²R, is a certain quantity of power, which the author proposes to call _power transformed into electricity_. When mechanical power is employed for producing a current by means of a magneto-electric or dynamo-electric machine--or, to use a better expression, by means of a _mechanical generator of electricity_--it is necessary in reality to expend a greater quantity of power than i²R in order to make up for losses which result either from ordinary friction or from certain electro magnetic reactions which occur. The ratio of the quantity, i²R, to the power, W, actually expended per unit of time is called the efficiency of the generator. Designating it by K, we obtain, W = i²R/K. It is very important to ascertain the value of this efficiency, considering that it necessarily enters as a factor into the evaluation of all the effects to be produced by help of the generator in question. The following table gives the results of certain experiments made early in 1879, with a Gramme machine, by an able physicist, M Hagenbach, Professor at the University at Basle, and kindly furnished by him to the author: Revolutions per minute 935 919.5 900.5 893 Total resistance in Siemens' units 2.55 3.82 4.94 6.06 Total resistance in absolute units 2.435 3.648 4.718 5.787 x10^9 x10^9 x10^9 x10^9 Intensity in chemical units 17.67 10.99 8.09 6.28 Intensity in absolute units 2.828 1.759 1.295 1.005 Work done i²R in absolute units 1948.6 1129.2 791.3 584.9 x10^7 x10^7 x10^7 x10^7 Work done i²R in kilogrammes 198.6 115.1 80.66 59.62 Power expended in kilogrammes 301.5 141.0 86.25 83.25 Efficiency, per cent. 65.9 81.6 93.5 71.6