Electricity for Boys

by James Slough Zerbe

6. The Calorimeter.—By using a coil of bare wire, immersed in paraffine oil, and then measuring the temperature by means of a thermometer.

Fig. 41. Acid Voltameter	Fig. 42. Copper Voltameter

7. The Light Method.—Lastly, by means of an electric light, which shows, by its brightness, a greater or less current.

The Preferred Methods.—It has been found that the first and second methods are the only[p. 57] ones which will accurately register current strength, and these methods have this advantage—that the chemical effect produced is not dependent upon the size or shape of the apparatus or the plates used.

How to Make a Sulphuric Acid Voltameter.—In Fig. [41] is shown a simple form of sulphuric acid voltameter, to illustrate the first method. A is a jar, tightly closed by a cover (B). Within is a pair of platinum plates (C, C), each having a wire (D) through the cover. The cover has a vertical glass tube (E) through it, which extends down to the bottom of the jar, the electrolyte therein being a weak solution of sulphuric acid. When a current passes through the wires (D), the solution is partially decomposed—that is, converted into gas, which passes up into the vacant space (F) above the liquid, and, as it cannot escape, it presses the liquid downwardly, and causes the latter to flow upwardly into the tube (E). It is then an easy matter, after the current is on for a certain time, to determine its strength by the height of the liquid in the tube.

How to Make a Copper Voltameter.—The second, or copper voltameter, is shown in Fig. [42]. The glass jar (A) contains a solution of copper sulphate, known in commerce as blue vitriol. A[p. 58] pair of copper plates (B, B') are placed in this solution, each being provided with a connecting wire (C). When a current passes through the wires (C), one copper plate (B) is eaten away and deposited on the other plate (B'). It is then an easy matter to take out the plates and find out how much in weight B' has gained, or how much B has lost.

In this way, in comparing the strength of, say, two separate currents, one should have each current pass through the voltameter the same length of time as the other, so as to obtain comparative results.

It is not necessary, in the first and second methods, to consider the shapes, the sizes of the plates or the distances between them. In the first method the gas produced, within a given time, will be the same, and in the second method the amount deposited or eaten away will be the same under all conditions.

Disadvantages of the Galvanoscope.—With the third method (using the galvanoscope) it is necessary, in order to get a positively correct reading instrument, to follow an absolutely accurate plan in constructing each part, in every detail, and great care must be exercised, particularly in winding. It is necessary also to be very careful in[p. 59] selecting the sizes of wire used and in the number of turns made in the coils.

This is equally true of the fourth method, using the electro-magnet, because the magnetic pull is dependent upon the size of wire from which the coils are made and the number of turns of wire.