The next thing was to enclose the filament in a glass globe in which not a trace of air would get near it. For if even the smallest quantity of air got near the filament while it was glowing, it would at once burn up. It was easy enough to pump all the air out of a glass globe, and leave only some other gas in the globe that would do no harm, but the difficulty was to mount a filament in the globe and to conduct a current through it from the outside. This was done by melting thin platinum wires into the glass. Platinum wires were the only wires which would not break the glass. For when platinum is heated it expands in the same way and to the same amount as glass, and therefore it does not break away from it. The filament was joined to the platinum wires by a special cement. After all this had been done, the air was pumped out, and the globe was melted off the pump at the lower end where the little point of glass is seen. The glass globe is called a bulb. It is provided with a brass collar from which two points stick out sideways. These points fit into recesses in another brass collar into which the wires bearing the electric current are led. This kind of joint is known as a bayonet joint. The current for the lamps is furnished from a special factory in which a steam-engine is made to turn an electric machine called a dynamo. Such a dynamo is constructed on the principle of the induction currents discovered by Faraday. Faraday’s whole life was spent in discovering new truths. He did not think first of all of the use his discoveries might be to mankind. He knew well that there were plenty of engineers and inventors ready to take advantage of his discoveries. It is very well that the discoverer and inventor should work hand in hand. The discoverer shows the way in which great achievements can be attained. The inventor makes these great practical achievements, and the good that results from them comes back to the discoverer in the gratitude of those who have been benefited by his discoveries and the happiness which followed from them. CHAPTER XIV MICHAEL FARADAY IT is strange that the man who did most for the progress of the science of electricity in the nineteenth century was the son of a London blacksmith. That man was Michael Faraday. He did not get much schooling, and what he did get was not more than reading, writing, and arithmetic. [Illustration: Fig. 66.--Michael Faraday.] His spare time was passed at home or in the streets. At the age of thirteen he was engaged as an errand boy to a bookbinder. After a year of running errands he was apprenticed to the trade of bookbinder, but the trade did not suit him at all, and all the time he could spare from it he spent in making chemical and electrical experiments. When he could get somebody to pay his admission fee, he attended lectures on these subjects.