In a thunderstorm we have the most spectacular display in lightning of a
violent and explosive rush of electrons (electricity) from one body to
another, from cloud to cloud, or to the earth. In this wonderful
photograph of an electrical storm note the long branched and undulating
flashes of lightning. Each flash lasts no longer than the one
hundred-thousandth part of a second of time.]
[Illustration: LIGHT WAVES
Light consists of waves transmitted through the ether. Waves of light
differ in length. The colour of the light depends on the wave-length.
Deep-red waves (the longest) are 7/250000 inch and deep-violet waves
1/67000 inch. The diagram shows two wave-motions of different
wave-lengths. From crest to crest, or from trough to trough, is the
length of the wave.]
[Illustration: THE MAGNETIC CIRCUIT OF AN ELECTRIC CURRENT
The electric current passing in the direction of the arrow round the
electric circuit generates in the surrounding space circular magnetic
circuits as shown in the diagram. It is this property which lies at the
base of the electro-magnet and of the electric dynamo.]
[Illustration: THE MAGNET
The illustration shows the lines of force between two magnets. The lines
of force proceed from the north pole of one magnet to the south pole of
the other. They also proceed from the north to the south poles of the
same magnet. These facts are shown clearly in the diagram. The north
pole of a magnet is that end of it which turns to the north when the
magnet is freely suspended.]