Nodular Deposit.

A deposit obtained in electroplating, characterized by irregular thickness; due to too low density of current.

Non-conductor.

A material that does not conduct electricity except with great difficulty; a substance of very high resistance.

Synonym--Insulator--Dielectric.

North Pole.

(a) The north-seeking pole of a magnet; the pole of a magnet which tends to point to the north, and whence lines of force are a.s.sumed to issue on their course to the other pole of the magnet.

(b) The North Pole of the earth. Treating the earth as a magnet, and accepting the above nomenclature the north pole should be termed the south pole. (See Austral Pole--Boreal Pole.)

393 STANDARD ELECTRICAL DICTIONARY.

North-seeking Pole.

The pole of a magnet which tends to point to the north; the north pole of a magnet.

Null Method.

Any method of obtaining measurements or comparisons, in which the measurement is correct when the deflection of the galvanometer or other indicator is zero, nought or null. The Wheatstone Bridge (see Bridge, Wheatstone) is an example of a null method.

Two obvious advantages attach to null methods in electric galvanometer work. One is that an uncalibrated galvanometer can be employed. The other is that a galvanometer of any high degree of sensitiveness can be employed, there being no restriction as to its fineness of winding or highness of resistance.

"Upper case Omega Graphic".

(Greek capital" Omega") symbol for megohm.

[Transcriber's note: Now used for ohms.]

"Lower case Omega Graphic".

(Greek omega) symbol for ohm.

[Transcriber's note: Now used for angular velocity, 2*PI*frequency.]

Occlusion.

An absorption of gases by metals. Palladium will, if used as the hydrogen evolving electrode in decomposing water, absorb 980 times its volume of hydrogen, which is said to be occluded. The metal may also be heated in hydrogen and allowed to cool therein, when occlusion occurs.

Platinum will occlude 4 times its volume of hydrogen; iron, 4.15 times its volume of carbon-monoxide; silver, 7 times its volume of oxygen.

Metals with occluded gases may serve as elements in a galvanic couple.

(See Gas Battery.) A metal expands in occluding a gas.

In the storage battery it is believed that occlusion plays a part, hydrogen and oxygen being respectively absorbed by the two sets of plates, and acting as they do in Groves' gas battery.

Oerstedt.

Name proposed for the unit of current strength, but not adopted. The ampere is the accepted name.

394 STANDARD ELECTRICAL DICTIONARY.

Oerstedt's Discovery.

Oerstedt discovered in 1820 that a magnetic needle tended to place itself at right angles to a current of electricity. This fundamental experiment is the basis of the galvanometer.

Fig. 251. OERSTEDT'S DISCOVERY.

Ohm.

The practical unit of resistance; 1E9 C. G. S. electro-magnetic units.

The legal ohm is the resistance of a mercury column 1 square millimeter in cross-sectional area and 106 centimeters in length. There has been considerable confusion, owing to inaccuracy in early determinations, in the valuation of the ohm. In this work the legal ohm is used. The different ohms will be found defined in their place. Resistance units of various names may also be consulted.

The following table gives the relative values of the different ohms.

Length of Mercury Board of Column in True B. A. Trade Legal Centimetre. Ohm. Ohm. Ohm. Ohm.

True Ohm, 106.24 1. 1.0128 .9994 1.0022 B. A. Ohm, 104.9 .9874 1. .9868 .9889 Board of Trade Ohm 106.3 1.00050 1.0133 1. 1.0028 Legal Ohm, 106.0 .9977 1.0112 .9971 1.

Ohmage.

The Resistance of a circuit expressed in ohms.

Ohm, B. A.

The British a.s.sociation unit of resistance; the resistance of a column of mercury 1 square millimeter in cross sectional area and 104.9 centimeters long; the B. A. Unit of Resistance.

Ohm, Board of Trade.

The approximate ohm as recommended by the British Board of Trade on the advice of a committee (Sir W. Thomson, Dr. J. Hopkinson, Lord Rayleigh and others). It is the resistance of a mercury column one square millimeter in section, and 106.3 centimeters long at 0? C. (32? F.)

Synonym--New Ohm.

395 STANDARD ELECTRICAL DICTIONARY.

Ohmic Resistance.

True resistance as distinguished from spurious resistance, or counter-electro-motive force.

Ohm, Legal.

The practical unit of resistance. The resistance of a column of mercury one square millimeter in cross-sectional area and 106 centimetres long at 0? C. (32? F.) The ohm used previously to 1884 is the B. A. Unit of Resistance, q. v.

One legal ohm = 1.0112 B. A. Units, and I B. A. Unit = 0.9889 legal ohm.

The resistance of a copper wire 1 foot long and 1/1000 inch in diameter is about 10 ohms. The resistance of 1 mile of iron wire 1/3 inch in diameter is about 10 ohms.