Generating Heat in a Wire.—When a current of electricity passes through a conductor, like a wire, more or less heat is developed in the conductor. This heat may be so small that it cannot be measured, but it is, nevertheless, present in a greater or less degree. Conductors offer a resistance to the passage of a current, just the same as water finds a resistance in pipes through which it passes. This resistance is measured in ohms, as explained in a preceding chapter, and it is this resistance which is utilized for electric heating.

Resistance of Substances.—Silver offers less resistance to the passage of a current than any other metal, the next in order is copper, while iron is, comparatively, a poor conductor.

The following is a partial list of metals, showing their relative conductivity:

From this table it will be seen that, for instance, iron offers six and a half times the resistance of silver, and that German silver has fifteen times the resistance of silver.

This table is made up of strands of the different metals of the same diameters and lengths, so as to obtain their relative values.

Sizes of Conductors.—Another thing, however, must be understood. If two conductors of the same metal, having different diameters, receive the same current of electricity, the small conductor will offer a greater resistance than the large conductor, hence will generate more heat. This can be offset by increasing the diameter of the conductor. The metal used is, therefore, of importance, on account of the cost involved.

Comparison of Metals.—A conductor of aluminum, say, 10 feet long and of the same weight as copper, has a diameter two and a quarter times greater than copper; but as the resistance of aluminum is 50 per cent. more than that of silver, it will be seen that, weight for weight, copper isp. 137 the cheaper, particularly as aluminum costs fully three times as much as copper.

The table shows that German silver has the highest resistance. Of course, there are other metals, like antimony, platinum and the like, which have still higher resistance. German silver, however, is most commonly used, although there are various alloys of metal made which have high resistance and are cheaper.

The principle of all electric heaters is the same,p. 138 namely, the resistance of a conductor to the passage of a current, and an illustration of a water heater will show the elementary principles in all of these devices.

A Simple Electric Heater.—In Fig. 96 the illustration shows a cup or holder (A) for the wire, made of hard rubber. This may be of such diameter as to fit upon and form the cover for a glass (B). The rubber should be ½ inch thick. Two holes are bored through the rubber cup, and through them are screwed two round-headed screws (C, D), each screw being 1½ inches long, so they will project an inch below the cap. Each screw should have a small hole in its lower end to receive a pin (E) which will prevent the resistance wire from slipping off.

The resistance wire (F) is coiled for a suitable length, dependent upon the current used, one end being fastened by wrapping it around the screw (C). The other end of the wire is then brought upwardly through the interior of the coil and secured in like manner to the other screw (D).

Caution must be used to prevent the different coils or turns from touching each other. When completed, the coil may be immersed in water, the current turned on, and left so until the water is sufficiently heated.

How to Arrange for Quantity of Currentp. 139 Used.—It is difficult to determine just the proper length the coil should be, or the sizes of the wire, unless you know what kind of current you have. You may, however, rig up your own apparatus for the purpose of making it fit your heater, by preparing a base of wood (A) 8 inches long, 3 inches wide and 1 inch thick. On this mount four electric lamp sockets (B). Then connect the inlet wire (C) by means of short pieces of wire (D) with all the sockets on one side. The outlet wire (E) should then be connected up with the other sides of the sockets by the short wires (F). If, now, we have one 16-candlepower lamp in one of the sockets, there is a half ampere going through the wires (C, F). If there are two lampsp. 140 on the board you will have 1 ampere, and so on. By this means you may readily determine how much current you are using and it will also afford you a means of finding out whether you have too much or too little wire in your coil to do the work.

An Electric Iron.—An electric iron is made in the same way. The upper side of a flatiron has a circular or oval depression (A) cast therein, and a spool of slate (B) is made so it will fit into the depression and the high resistance wire (C) is wound around this spool, and insulating material, such as asbestos, must be used to pack around it. Centrally, the slate spool has an upwardly projecting circular extension (D) which passes through the cap or cover (E) of the iron. The wires of the resistance coil are then broughtp. 141 through this circular extension and are connected up with the source of electrical supply. Wires are now sold for this purpose, which are adapted to withstand an intense heat.

The foregoing example of the use of the current, through resistance wires, has a very wide application, and any boy, with these examples before him, can readily make these devices.

Thermo Electricity.—It has long been the dream of scientists to convert heat directly into electricity. The present practice is to use a boiler to generate steam, an engine to provide the motion, and a dynamo to convert that motion into electricity. The result is that there is loss in the process of converting the fuel heat into steam; loss to change the steam into motion, and loss top. 142 make electricity out of the motion of the engine. By using water-power there is less actual loss; but water-power is not available everywhere.

Converting Heat Directly Into Electricity.—Heat may be converted directly into electricity without using a boiler, an engine or a dynamo, but it has not been successful from a commercial standpoint. It is interesting, however, to know and understand the subject, and for that reason it is explained herein.

Metals; Electric Positive-Negative.—To understand the principle, it may be stated that all metals are electrically positive-negative to each other. You will remember that it has hereinbefore been stated that if, for instance, iron and copper are put into an acid solution, a current will be created or generated thereby. So with zinc and copper, the usual primary battery elements. In all such cases an electrolyte is used.

Thermo-electricity dispenses with the electrolyte, and nothing is used but the metallic elements and heat. The word thermo means heat. If, now, we can select two strips of different metals, and place them as far apart as possible—that is, in their positive-negative relations with each other, and unite the end of one with one end of other by means of a rivet, and then heat the riveted ends, a current will be generated inp. 143 the strips. If, for instance, we use an iron in conjunction with a copper strip, the current will flow from the copper to the iron, because copper is positive to iron, and iron negative to copper. It is from this that the term positive-negative is taken.

The two metals most available, which are thus farthest apart in the scale of positive-negative relation, are bismuth and antimony.

In Fig. 101 is shown a thermo-electric couple (A, B) riveted together, with thin outer ends connected by means of a wire (C) to form a circuit. A galvanometer (D) or other current-testing means is placed in this circuit. A lamp is placed below the joined ends.

Thermo-Electric Couples.—Any number of these couples may be put together and joined at each end to a common wire and a fairly large flow of current obtained thereby.

One thing must be observed: A current willp. 144 be generated only so long as there exists a difference in temperature between the inner and the outer ends of the bars (A, B). This may be accomplished by water, or any other cooling means which may suggest itself.

p. 145