Units Of Measurement

The three most commonly used units are:

I. The unit of current, called the Ampere; II. The unit of potential, called the Volt; III. The unit of resistance, called the Ohm.

For some purposes these quantities are subdivided, thus in telegraphy the practical unit of current is the milli-ampere, i.e., one-thousandth of an ampere. In some cases it is convenient to use multiples; insulation resistances are often expressed in terms of megohms, i.e., a million ohms. The most commonly used multiples are the following:

1

Megohm =

106 ohms

= 1 million ohms,

1

Microhm =

10-6 ohm

= 1 millionth of an ohm,

1

Kilowatt =

103 watts

= 1,000 watts,

1

Micro-ampere =

10-6 ampere

= 1 millionth of an ampere.

Ohm's Law

For steady currents the three quantities - current, potential, and resistance - are connected together by the relation discovered by Dr. Ohm, and called Ohm's Law. This law is stated thus

C = E / R; where C = current (amperes);

E = difference of potential (volts); R = resistance opposing the current (ohms). All the units in scientific work are defined in terms of the fundamental units, which are Unit of length = 1 centimeter. " " mass = 1 gram. " " time = 1 second. These are spoken of as the C.G.S. units, and in the actual determination of a standard ohm attempts have been made to obtain the scientific value as closely as possible. The first unit used as a standard was the British Association or B.A. unit coil. Messrs. Siemens also introduced a standard ohm, but both of these units differed from the true ohm as well as from each other. In order to avoid the consequent confusion, an international congress was held at Paris in 1893 to decide upon the standard values to be adopted.