Carbon rods and plates are easily drilled with an ordinary hand drill. Carbon is quite brittle and breaks easily, therefore only very light pressure should be used.
While zinc rods and plates may also be purchased they are easily made by the experimenter who possesses a little ingenuity. The melting point of zinc is quite low. It can be melted in a small iron pot and cast into the form of rods or plates in plaster-of-Paris moulds. Plates may also be cut out of heavy sheet zinc.
[Illustration: FIG. 28.—A Simple Home-made Cell.]
Ordinary jelly-glasses, tumblers, fruit jars, etc., make good jars for small cells. The tops of fruit jars and batteries can be cut off so as to make the opening larger.
The cutting can be done with an ordinary glass cutter or by filling a scratch completely around the jar or bottle, at the place it is desired to cut it off, with a three cornered file. If a hot poker or wire is then held against the scratch it will commence to crack along the line and follow the hot poker as it is drawn around.
[Illustration: FIG. 29.—A Home-made Battery having two Carbon Plates with a Zinc Rod between.]
Figure 28 shows a simple arrangement consisting of a carbon and a zinc plate mounted upon a wooden strip. The strip is used to support the plates and rests across the top of the jar so that the plates hang below in the solution. Most chemicals attack wood and for that reason it is well to dip the strip in some hot paraffin. The carbon and zinc plates are fastened on opposite sides of the wooden strip by means of a round headed screw and a washer. A wire lead should be placed under the washer on each plate. If the screw and washer are then smeared with some hot paraffin or vaseline they will be protected from corrosion.
Care should be used so that the two screws employed to fasten the plates to the strip do not touch each other in the wood. If they should, the battery will be "short circuited" and the current will flow through the screws instead of the wires.
Figure 29 shows an arrangement consisting of two carbon plates mounted upon a wooden strip. The zinc element consists of a rod set in a whole in the strip between the two carbon plates.
It will be found that two carbon plates will form a better cell than one with only one plate or rod.