Elementary lessons in electricity & magnetism by Thompson Silvanus Phillips 1851-1916

Author:Thompson, Silvanus Phillips, 1851-1916. [from old catalog]
Language: eng
Format: epub
Tags: Magnetism
Publisher: London, Macmillan and co.
Published: 1884-03-25T05:00:00+00:00

322. Electromagnetic Units.—The preceding magnetic units give rise to the following set of electrical units, in which the strength of currents, etc., are expressed in magnetic measure. The relation of this "electromagnetic" set of units to the "electrostatic'' set of units of Art. 257 is explained in Art

365.

Unii Strength of Current, —^A current has unit strength when one centimetre length of its circuit bent into an arc of one centimetre radius (so as to be always one centim. away from the magnet-pole) exerts a force of one dyne on a unit magnet-pole placed at the centre (Art. 196).

Unit of Quantity of Electricity is that quantity which is conveyed by unit current in one second.

Unit of Difference of Potential (or of JS/ectromottve-ffrce), Potential is work done on a unit of electricity; hence unit difference of potential exists between two points when it requires the expenditure of one erg of' work to bring a unit of -^ electricity from one point to the other against the electric force.

Unit of /Resistance. —A conductor possesses unit resistance when unit difference of potential between its ends causes a current of unit strength {ue. one unit of quantity per second) to flow through it.

323. Practical Units.—Several of the above "absolute** units would be inconveniently large and others inconveniently small for practical use. The following are therefore chosen instead, as electromagnetic units :—

Electromotiveforce, —^The Volt, = 10^ absolute units (being a little less than the E.M.F. of one Daniell's cell);

/Resistance. —The Ohm, = 10* absolute units of resistance (and theoretically the resistance represented by the velocity of one earth-quadrant per second). {See Art 364.)

Current. —As a practical unit of current, that frimished by a potential of one volt though one ohm is taken, being

10 " of an absolute (electro-magnetic) unit of current, and is known as one Ampere (formerly one " weber '*).

Quantity. —The Coulomb, = 10"" ^ absolute units of quantity of the electromagnetic system.

Capacity. —^The F^lrad, = io"~* (or one one-thousand-millionth) of alisolute unit of capacity.

ELEMENTARY LESSONS ON [chap. v.

Seeing, however, that quantities a million times as great as some of these, and a million times as small as some, have to be measured by electricians, the prefixes mega- and micro- are sometimes used to signify respectively " one million ** and " one-millionth part.** Thus a megohm is a resistance of one million ohms, a microfarad a capacity of ^ .000.000 ®^ * ferad, etc. The prefix milli- is frequently used for "one-thousandth part ;** thus a miUi-amplre is the thousandth part of one ampere.

This system of " practical" units was devised by a committee of the British Association, who also determined the value of the **ohm'* by experiment, and constructed standard resistance coils of german-silver, called **B. A. Units" or **ohms.'* The " practical ** system may be regarded as a system of units derived not from the fundamental units of centimetre^ gramme^ and second^ but from a system in which, while the unit of time remains the second, the units oT length and mass are respectively the earth-quadrant and 10—^^ gramme.

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