The reason why an _open_ arc lamp needs to be "retrimmed" oftener than the _enclosed_ lamp, that is, have new carbons placed in it, is because the carbons "burn" freely, that is unite with the oxygen of the air. In the enclosed arc lamp, the supply of oxygen in the inner globe is limited and is soon consumed, therefore the carbons last many times longer in such lamps. Some carbon rods have soft cores containing calcium salts. These vaporize in the arc producing the _flaming arc light_ of a bright yellow color, and give more light than the ordinary lamp. Important Topics 1. Heat effects of electric currents, uses and applications. 2. Computation of the heat developed in a circuit. Three formulas. 3. Electric lamps; incandescent and arc; construction, uses, efficiency. Exercises 1. Sketch a circuit containing 10 incandescent lamps in parallel. If each lamp when hot has a resistance of 220 ohms, and the E.M.F. is 100 volts, what current will flow? 2. What will it cost to use these lights for 3 hours a day for 30 days at 10 cents a kilowatt hour? 3. How much heat will these lamps produce per minute? 4. How could you connect 110-volt lamps to a street car circuit of 660 volts? Explain this arrangement and draw a diagram. 5. A certain arc lamp required 10 amperes of current at 45 volts pressure. What would it cost at 10 cents per kilowatt hour if used 3 hours a day for 30 days? 6. Show a diagram of 3 arc lamps in series. If each takes 45 volts and 10 amperes, how much E.M.F. and current will they require? 7. If an electric toaster uses 5 amperes at 115 volts, how much heat will this develop in half an hour? 9. How much heat is developed in an electric toaster in 2 minutes, if it uses 5 amperes at 100 volts? 10. How many B.t.u.'s are given off in an electric oven that takes 10 amperes at 110 volts for 1 hour? (1 B.t.u. equals 252 calories.) 11. An electric heater supplies heat at the rate of 700 B.t.u.'s an hour. How much power does it require? 12. How many watts are required to operate 120 incandescent lamps in parallel if each takes 0.5 amperes at 110 volts? 13. An electric lamp takes 12 amperes at a P.D. of 110 volts. How many B.t.u.'s are radiated from it each second? How many calories? 14. If a 110-volt incandescent lamp is submerged for 10 minutes in 400 gr. of cold water while a current of 0.5 amperes is flowing, how many degrees centigrade will the water be warmed? 15. In an electric furnace a current of 3000 amperes is used at a P.D. of 10 volts. Find the heat developed in 1 minute. 16. How many candle power should a 20-watt tungsten lamp give if its efficiency is one watt per candle power? 17. What is the "efficiency" of a 40-watt tungsten lamp if it gives 34 candle power? Review Outline: Current Electricity Produced by--Chemical action; voltaic and storage cells.