[Illustration: FIG. 95. Parts of electric iron.] =179. Charcoal Irons.= Charcoal is no longer used for heating irons. It makes too much dirt. Difficulty is found, also, in keeping charcoal irons at a constant temperature. =180. Electric Irons.= An electric iron (Fig. 95) is made up of a heavy nickel-plated base, a block of iron which holds the heat, and a heating unit of small wires, or a plate, thru which the current passes, meeting resistance. Since resistance against the flow of an electric current produces heat, the iron is heated. It has a handle and shell covering the heating unit to protect the hand and prevent loss of heat thru the top. Getting electric irons too hot injures the heating unit, as electricity can heat metals so hot that they melt. Excessive heat may disconnect the circuit by burning the wires in the iron, or it may melt the metal so as to form a short circuit. Always follow exactly the directions for connecting and disconnecting the iron with the current. Some say disconnect at the plug between iron and cord, or others the plug placed near the socket (Fig. 95-_a_). The weakest part in irons is likely to be in the attachment plug. When connecting the plug to the iron, be sure to get it back in place each time. A plug that does not fit well into place may cause sparking and develop sufficient heat to burn off the insulation from the cord, if not the fuses of the system to which the iron is attached. Never attach an iron to a lighting system without making sure that the iron is made to be operated on the voltage of the current to which is is connected. If it is not the same, attaching the iron may either burn out the fuses of the lighting system, or ruin the iron. [Illustration: FIG. 95-_a_. Connecting plug for electric attachment.] Operate the iron at a good temperature for ironing, and take care to keep it from getting hotter than is required. =181. Gas Irons.= Gas irons are attached to a tube leading from a gas pipe. There is a burner inside the iron which is generally a straight rod with perforations in it for the escape of the mixture of gas and air. The air mixes with the gas at a point near where the gas pipe enters the iron. The principle of heating an iron is the same as the heating of a gas stove (Fig. 96). The burner in the iron is lighted, and as soon as it has heated the iron, the ironing can proceed. The only difficulties encountered in using this kind of an iron are that a quick, jerky stroke may blow out the flame, and if the work is being done in a drafty place, the iron may not heat evenly. These difficulties can be overcome, however. The person using the iron can learn to use a stroke which will be rapid and still not put out the flame. The ironing board may be protected from drafts. A gas iron is safe and practical. It is easily controlled by the valve admitting the gas.