The fact that it is a reverse current is shown by the actions of the galvanometer for it will be noticed that the needle swings in the opposite direction this time.
It will also be noticed that no current is produced when the coil and magnet are stationary. Current is only generated when the coil and magnet are approaching one another or moving apart suddenly.
This is because it is only then that the magnetic field is changing. The field is strongest nearest the magnet, and therefore if either the magnet or the coil of wire is moved, the strength of that part of the field which intersects the coil is changed. Induced currents can only be generated by a changing magnetic field.
| [1] | See chapter on Measuring Instruments. |
CHAPTER VI ELECTRICAL UNITS
The Ampere
There are certain terms used in the electrical field to distinguish various properties and qualities of the electrical current with which it is well for the young experimenter to acquaint himself.
One of the first units usually required, in order to make intelligent comparisons, is a unit of measure. The quart is the unit of measure commonly applied to liquids and is based upon the amount of space occupied by a certain volume. The pound is a unit of weight which determines a certain amount of any substance by comparing the force which gravity exerts in pulling it to the earth with the same effect of gravity on another standard "weight."
Electric current is invisible and weightless, and for these and other reasons cannot be measured by the quart or weighed by the pound. The only way that it can be measured is by means of some of the effects which it produces. Either the chemical, electro-magnetic, or the heating effects may be made the basis of a system of measurement.