Measurement of a Galvanometer Resistance-Thom- Measurement of a Battery Resistance-Mance's 78. The British Association Wire Bridge Measurement of Electrical Resistance Relation between the Quantity of Electricity which passes through a Galvanometer and the initial Angular Velocity produced in the Needle Work done in turning the Magnetic Needle through Electrical Accumulators or Condensers 82. Measurement in Absolute Measure of the Capacity of a Y. Experimental Laws of Electromagnetic Induction (1) The quantity of Electricity traversing the Secondary is directly proportional to the (2) The quantity of Electricity traversing the г. Comparison of a Coefficient of Mutual Induction with the Capacity of a Condenser and the. Product of two Re- Determination of the Dip by the Earth Inductor Measurement of the Magnetic Induction at any point UNIVER D PRACTICAL PHYSICS. CHAPTER I. PHYSICAL MEASUREMENTS. THE greater number of the physical experiments of the present day and the whole of those described in this book consist in, or involve, measurement in some form or other. Now a physical measurement—a measurement, that is to say, of a physical quantity-consists essentially in the comparison of the quantity to be measured with a unit quantity of the same kind. By comparison we mean here the determination of the number of times that the unit is contained in the quantity measured, and the number in question may be an integer or a fraction, or be composed of an integral part and a fractional part. In one sense the unit quantity must remain from the nature of the case perfectly arbitrary, although by general agreement of scientific men the choice of the unit quantities may be determined in accordance with certain general principles which, once accepted for a series of units, establish certain relations between the units thus chosen, so that they form members of a system known as an absolute system of units. For example, to measure energy we must take as our unit the energy of some body under certain conditions, but when we agree that it shall always be the energy of a body on which a unit force has acted through unit space, our choice has been exercised, and the unit of energy is no longer arbitrary, but defined, as soon as the units of force and space are agreed upon; we have thus substituted the right of selection of the general principle for the right of selection of the particular unit. We see, then, that the number of physical units is at least as great as the number of physical quantities to be measured, and indeed under different circumstances several different units may be used for the measurement of the same quantity. The physical quantities may be suggested by or related to phenomena grouped under the different headings of Mechanics, Hydro-mechanics, Heat, Acoustics, Light, Electricity or Magnetism, some being related to phenomena on the common ground of two or more such subjects. We must expect, therefore, to have to deal with a very large number of physical quantities and a correspondingly large number of units. The process of comparing a quantity with its unit-the measurement of the quantity-may be either direct or indirect, although the direct method is available perhaps in one class of measurements only, namely, in that of length measurements. This, however, occurs so frequently in the different physical experiments, as scale readings for lengths and heights, circle readings for angles, scale readings for galvanometer deflections, and so on, that it will be well to consider it carefully. The process consists in laying off standards against the length to be measured. The unit, or standard length, in this case is the distance under certain conditions of temperature between two marks on a bar kept in the Standards Office of the Board of Trade. This, of course, cannot be moved from place to place, but a portable bar may be obtained and compared with the standard, the difference between the two being expressed as a fraction of the standard. Then we may apply the portable bar to the length to be measured, determining the number of times the length of the bar is contained in the given length, with due allowance for temperature, and |