30. Electric Welding.—Nothing is more common in electrical matters than heat produced by poor contacts. In this laboratory are two chandeliers, each controlled by a wall switch. After the current has been on the chandeliers for half an hour you will always find one of those wall switches warm, while the other is not perceptibly warmer than other objects in the room. The explanation is that there is poor contact in one of them. When two metal conductors touch one another at a mere point the electric current, in passing from one of these conductors to the other across such a narrow bridge, meets resistance and develops heat—sometimes heat enough to fuse the point, and either break the contact, or, what is more likely, start a minute arc at that point. In some cases this makes the apparatus dangerously hot, and in other cases it bridges the gap with a broader and better contact—a true electric weld. Electric welding is applied to everything, from chicken fence to railway rails. Enormously large currents are used for the purpose, in some cases as high as 50,000 amperes being employed. The rails of railroads are welded end to end by a current of several thousand amperes sent through the joint by perhaps two or three volts. The joint heats and fuses together merely because the poor contact offers resistance to this enormous current.
IX
LIGHTING A SUMMER CAMP BY ELECTRICITY
Summer had arrived. The Science Club had held its last meeting for the season. Harold had engaged three other boys to spend the summer at the farm. I had the roof of an old mill reshingled and gave it to them for a camp. They were to make it over inside. I sent the boys to the country as early as it was possible for them to get away. It would be six weeks later before I could follow them.
Fig. 107
When I did arrive I found they had elaborate schemes indeed. The first floor of the mill had been partitioned off into rooms, as shown in diagram ([Fig. 107]), a, b, c and d being bedrooms; e was a wash room, the like of which has never been seen before. It had not occurred to me that the mill pond m, which came to the very corner of the building, would furnish the boys a complete system of city water-works. At g, in the corner of this room, they had cut a hole in the floor and nailed slats across upon the under side of the timbers, making a depressed floor for a shower bath. This was directly over a stream of water which issued from the mill pond. Hanging from the ceiling over this spot was the nozzle of a garden hose. The other end of this hose ran into the mill pond. The nozzle was capable of delivering either a stream or a shower, according to which way it was twisted in its socket. It was also capable of shutting off entirely the flow of water. The boys asked me to hold my hand in the shower, and to my astonishment it was warm. "What, pray, is your heating system?" I inquired. They invited me to go and see. Moored outside in the mill pond at the corner of the building was our motor boat, which the boys were allowed to use freely and which they understood as well as any one.
