John Ambrose Fleming

“ As a unit of electromotive force, the International Volt, which is the electromotive force that, steadily applied to a conductor whose resistance is one international ohm, will produce a current of one international ampere. ”

“ Similarly we may select for more special measurements any arbitrary electric current, electromotive force, or resistance, and call them our units. The progress of knowledge, however, is greatly assisted if all the measurable quantities are brought into relation with each other by so selecting the units that they are related in the most simple manner, each to the other and to one common set of measurable magnitudes called the fundamental quantities. ”

“ The modern absolute system of physical measurement is founded upon dynamical notions, and originated with C. F. Gauss. We are for the most part concerned in studying motions in nature; and even when we find bodies at rest in equilibrium it is because the causes of motion are balanced rather than absent. Moreover, the postulate which lies at the base of all present-day study of physics is that in the ultimate issue we must seek for a mechanical explanation of the facts of nature if we are to reach any explanation intelligible to the human mind. ”

“ Accordingly a truly natural system of physical units would be one which was based upon the electron, or a multiple of it, as a unit of electric quantity, the velocity of light or fraction of it as a unit of velocity, and the mass of an atom of hydrogen or multiple of it as a unit of mass. ”

“ The common British unit is the “poundal,” the force which in one second retards or accelerates the velocity of a mass of one pound by one foot per second. The metric (and scientific) unit, named the “dyne,” is derived from the centimetre, gramme, and second. The poundal and dyne are related as follows:—1 poundal= 13,825·5 dynes.
A common unit of force, especially among engineers, is the “weight of one pound,” by which is meant the force equivalent to the gravitational attraction of the earth on a mass of one pound. This unit obviously depends on gravity ”

“ Accordingly the root of all science is the knowledge of the laws of motion, and the enunciation of these laws by Newton laid the foundation of a more exact knowledge of nature than had been possible before. Our fundamental scientific notions are those of length, time, and mass. ”

“ Energy of work is measured by force acting over a distance. The scientific unit is the “erg,” which is the energy expended when a force of one dyne acts over one centimetre. This unit is too small for measuring the quantity of energy associated, for instance, with engines; for such purposes a unit ten-million times as great, termed the “joule,” is used. ”

“ Suppose such a sphere to be charged and discharged rapidly with electricity from any source, such as a battery. It would take electricity from the source at a certain rate, and would in fact act like a resistance in permitting the passage through it or by it of a certain quantity of electricity per unit of time. If K is the capacity and n is the number of discharges per second, then nK is a quantity of the dimensions of an electric conductivity, or of the reciprocal of a resistance. ”

“ The choice is decided by the nature of the quantities themselves. Since resistance is a permanent quality of a substance, it is possible to select a certain piece of wire or tube full of mercury, and declare that its resistance shall be the unit of resistance, and if the substance is permanent we shall possess an unalterable standard or unit of resistance. For these reasons the practical unit of resistance, now called the international ohm, has been selected as one of the above three electrical units. ”

“ The British Association Committee defined the strength of a magnetic pole by reference to the mechanical stress between it and another equal pole: hence the British Association unit magnetic pole is a pole which at a distance of one centimetre attracts or repels another equal pole with a force of one dyne. This, we have seen, is an imperfect definition, because it omits all reference to the permeability of the medium in which the experiment takes place ”

“ In British engineering practice the common unit of power is the “horse-power” (HP ) , which equals 550 foot-pounds performed per second, or 33,000 foot-pounds per minute; its equivalent in the metric system is about 746 watts, the ratio varying, however, with gravity.
Units of Heat.—In studying the phenomena of heat, two measurable quantities immediately present themselves:— (1) temperature or thermal potential, and (2) quantity of heat. ”

“ The practical electrical engineer, up to the present, prefers to use one ampere-turn as his unit of magnetomotive force, and one line of force as the unit of magnetic flux, equal respectively to 10/4π times and 1 times the C.G.S. absolute units. Very frequently the “kiloline,” equal to 1000 lines of force, is now used as a unit of magnetic flux. ”

“ If we consider two very small spheres placed with centres 1 centimetre apart in air and charged with equal quantities of electricity, then if the force between these bodies is 1 dyne each sphere is said to be charged with 1 unit of electric quantity on the electrostatic system. Again, if we consider two isolated magnetic poles of equal strength and consider them placed 1 centimetre apart in air, then if the force between them is 1 dyne these poles are said to have a strength of 1 unit on the electromagnetic system. ”

“ As a unit of resistance, the International Ohm, which is based upon the ohm equal to 109 units of resistance of the C.G.S. system of electromagnetic units, and is represented by the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice 14·4521 grammes in mass, of a constant cross-sectional area and of the length of 106·3 cm. ”

“ We may, for instance, .choose a certain length, a certain volume, a certain mass, a certain force or power as our units of length, volume, mass, force or power, which have no simple or direct relation to each other. ”

“ There is nothing to prevent us from selecting these fundamental quantities, in terms of which other like quantities are to be expressed, in a perfectly arbitrary and independent manner, and as a matter of fact this is what is generally done in the early stages of every science. ”

“ On this system the unit of force is the dyne and the unit of work the erg. The dyne is the uniform force which when acting on a mass of 1 gramme for 1 second gives it a velocity of 1 centimetre per second. The erg is the work done by 1 dyne when acting through a distance of 1 centimetre in its own direction. ”

“ On the C.G.S. or British Association system, if a magnetic filament has a pole strength m—that is to say, if it has a magnetization I, and a section s, such that Is equals m—then it can be shown that the total flux emanating from the pole is . The factor , in consequence of this definition, makes its appearance in many practically important expressions. ”

“ The most frequent uses of this engineer's unit are to be found in the expressions for pressure, especially in the boilers and cylinders of steam engines, and in structures, such as bridges, foundations of buildings, &c. ”