## Motor-dynamo

A motor and a dynamo having their armatures connected together, whereby the motor is driven by the dynamo, so as to change the current into a different voltage and amperage.

## Motor-transformer

A motor which delivers the current like a generator.

## Needle

A bar magnet horizontally poised on a vertical pivot point, like the needle of a mariner's compass.

## Negative Electricity

Amber, when rubbed, produces negative electricity. A battery has positive as well as negative electricity.

## Negative Element

That plate in the solution of a battery cell which is not disintegrated.

## Normal

The usual, or ordinary. The average. In a current the regular force required to do the work.

## North Pole, Electric

The term applied to the force located near the north pole of the globe, to which a permanent magnet will point if allowed to swing freely.

## O

Abbreviation for Ohm.

## Ohm

The unit of resistance. Equal to the resistance of a column of mercury one square millimeter in cross section, and 106.24 centimeters in length.

## Ohm's Law

It is expressed as follows:

1. The current strength is equal to the electro-motive force divided by its resistance.
2. The electro-motive force is equal to the current strength multiplied by the resistance.
3. The resistance is equal to the electro-motive force divided by the current strength.

In a motor an excess of mechanical work which causes the armature to turn too slowly and produces heat.

## Phase

One complete oscillation. The special form of a wave at any instant, or at any interval of time.

## Plate, Condenser

In a static machine it is usually a plate of glass and revoluble.

## Plate, Negative

The plate in a battery, such as carbon, copper or platinum, which is not attacked by the solution.

## Plating, Electro-

The method of coating one metal with another by electrolysis.

## Polarity

The peculiarity, in a body, of arranging itself with reference to magnetic influence.

## Parallel

When a number of cells are coupled so that their similar poles are grouped together. That is to say, as the carbon plates, for instance, are connected with one terminal, and all the zinc plates with the other terminal.

## Polarization

When the cell is deprived of its electro-motive force, or any part of it, polarization is the result. It is usually caused by coating of the plates.

## Porosity

Having small interstices or holes.

## Positive Current

One which deflects a needle to the left.

## Positive Electricity

Any current flowing from the active element, such as zinc, in a battery. The negative electricity flows from the carbon to the zinc.

## Potential, Electric

The power which performs work in a circuit.