2. Draw a diagram of an electric bell circuit at your home. Give the location of the electric bell, the electric generator and the push-button. Show the connecting wires, and explain briefly how the circuit is operated.
3. Represent some other electric circuit, naming the generator and other devices in the circuit.
4. Look up the work of Volta and Galvani and write a statement of the electrical discoveries and inventions made by them.
(2) The Voltaic Cell and its Action
241. The simple voltaic cell consists of a strip of copper and a strip of zinc placed in dilute sulphuric acid. (See Fig. 219.) A short time after placing the plates in the acid, bubbles of a gas (hydrogen) appear on the surface of the zinc. These bubbles increase in size and some rise to the surface of the liquid. Nothing appears upon the copper plate. If the tops of the plates are connected by a wire, an electric current is set up through the wire and the cell, and bubbles of gas also appear upon the copper as well as on the zinc. In a short time the surface of the copper becomes coated with bubbles and the current becomes much weaker. If the plates are left in the acid for some time the zinc is found to be eaten away, having been dissolved in the acid through chemical action. The copper, however, remains practically unaffected.
Fig. 219.—A simple voltaic cell.
242. How the Current is Produced.—To maintain the electric current a continual supply of energy is required. This is furnished by the chemical action of the acid upon the zinc. The chemical action is in several respects like combustion or burning, by means of which chemical energy is transformed into heat energy. In the voltaic cell the chemical action of the acid upon the zinc transforms chemical energy into electrical energy. The E.M.F. or difference of potential may be considered as originating at the surface of the zinc where the chemical action takes place. At this point the zinc has the lower and the liquid in contact with it the higher potential. The molecules of the acid are believed to be separated or broken up into two parts called ions; one ion, the SO4 or sulphion, combines with the zinc forming zinc sulphate, the other, or hydrogen (H) ion, passes over to the copper plate, and accumulates on the surface of this plate giving it a positive charge. It is therefore called the positive ion. The sulphion ion, or SO4 ion, carries a negative charge to the zinc. It is therefore called the negative ion.
243. The Direction of the Current.[K]—Beginning at the surface of the zinc the direction of the movement of positive electricity may be traced through the liquid to the copper plate, to the wire, to the zinc plate, to the starting point, thus completing the electric circuit. When the circuit is closed it is found that the movement of electricity starts in all parts of the circuit at practically the same instant.
