This section is from the book "Scientific American Reference Book. A Manual for the Office, Household and Shop", by Albert A. Hopkins, A. Russell Bond. Also available from Amazon: Scientific American Reference Book.

The Ohm is represented by the resistance offered by a column of mercury - at the temperature of melting ice - 14.4521 grams in mass, of a constant cross-sectional area, and of a length of 106.3 centimeters.

The Ampere is represented by the unvarying electric current which, when passed through a solution of nitrate of silver in water, deposits silver at the rate of 0.001118 of a gram per second.

The Volt is the electrical pressure which, if steadily applied to a conductor whose resistance is 1 ohm, will produce a current of 1 ampere, and which is represented by 0.6974, or 1000/1434 of the electrical pressure between the poles of the voltaic cell, known as Clark's cell, at a temperature of 15° C. (59° F.).

As in many of the older books and early papers dealing with electrical matters the older system of units is used, the following table will be useful for ascertaining the relative values of the quantities expressed:

System. | True Ohm. | Legal Ohm. | B.A. Ohm. | Siemens Ohm. |

True Ohm..... | 1.0000 | 1.0025 | 1.0138 | 1.0630 |

Legal Ohm.... | 0.9975 | 1.0000 | 1.0113 | 1.0600 |

B.A. Ohm..... | 0.9863 | 0.9889 | 1.0000 | 1.0482 |

Siemens Ohm.. | 0.9408 | 0.9434 | 0.9540 | 1.0000 |

The quantity of electricity that flows per second past a cross-section of a conductor carrying a current of one ampere is a Coulomb.

The practical unit is the quantity that flows per hour, and is measured in ampere-hours.

Unit oF Capacity: The Farad. - The capacity of two conductors insulated from each other is the number of coulombs of electricity required to be given to one conductor, the other being supposed at zero potential, to produce a difference of pressure of 1 volt between the two. The unit of capacity is called a "farad," and two conductors arranged in a form known as a condenser of 1 farad capacity would be raised to a difference of pressure of 1 volt by a charge of 1 coulomb of electricity. The practical unit used, however, has a capacity one-millionth of a farad - i.e., a microfarad.

When a power of one watt is being developed, the work done per second is sometimes called a "Joule." Hence, one joule equals 0.7375 foot-lb., and

1 watt-second = 1 joule.

1 watt-minute =60 joules.

1 horse-power hour = 1,980,000 foot-lbs.

1 horse-power hour = 2,685,600 joules.

(W. E. Ayrton.)

A "watt" is the power developed in a circuit when one ampere flows through it, and when the potential difference at its terminals is one volt; hence the number of watts developed in any circuit equals the product of the current in amperes flowing through it into the potential difference at its terminals in volts. Therefore

1 watt is the power developed when 44.25 foot-lbs. of work are done per minute.

1 watt is the power developed when 0.7375 foot-lb. of work is done per second.

1 watt equals 1/746th of a horse-power.

(W. E. Ayrton.)

The amount of heat required to raise 1 kilogram of water 1° C. is the unit of heat employed on the Continent.

1 calorie = 4,200 joules = 42 X 109 ergs.

1 joule = 0.000238 calories.

Induction: The Henry. - The induction in a circuit when the difference of electrical pressure induced in the circuit is 1 volt, while the inducing current varies at the rate of 1 ampere per second, is called a "Henry."

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