2. Resistance furnaces. In the other type of furnace the heat is generated by the resistance offered to the current in its passage through the furnace. In its simplest form it may be represented by Fig. 68. The furnace is merely a rectangular box built up of loose bricks. The electrodes E, each consisting of a bundle of carbon rods, are introduced through the sides of the furnace. The materials to be heated, C, are filled into the furnace up to the electrodes, and a layer of broken coke is arranged so as to extend from one electrode to the other. More of the charge is then placed on top of the coke. In passing through the broken coke the electrical current encounters great resistance. This generates great heat, and the charge surrounding the coke is brought to a very high temperature. The advantage of this type of furnace is that the temperature can be regulated to any desired intensity.
EXERCISES
1. Why does charcoal usually burn with no flame? How do you account for the flame sometimes observed when it burns?
2. How do you account for the fact that a candle burns with a flame?
3. What two properties must the mantle used in the Welsbach lamp possess?
4. (a) In what respects does the use of the Welsbach mantle resemble that of lime in the calcium light? (b) If the mantle were made of carbon, would it serve the same purpose?
5. Would anthracite coal be suitable for the manufacture of coal gas?
6. How could you prove the formation of carbon dioxide and water in the combustion of illuminating gases?
7. Suggest a probable way in which natural gas has been formed.
8. Coal frequently contains a sulphide of iron. (a) What two sulphur compounds are likely to be formed when gas is made from such coal? (b) Suggest some suitable method for the removal of these compounds.