In replacing the bulbs care must be taken to see that the voltage is that suited to the battery. The voltage is usually stamped on the lamp base or marked on the bulb. In case a lamp intended for a single cell is used with a battery of three or four cells, the lamp filament will soon be destroyed. The reverse will be true should a lamp intended for a battery be used with a single cell. The single cell giving not much more than a volt of electromotive force will not send sufficient current through the lamp filament to render it incandescent. =The Electric Flat-iron.=--The changes that have been made in domestic appliances by the extended use of electricity have brought many innovations but none are more pronounced than the improvements made in the domestic flat-iron. It was the first of the household heating devices to receive universal recognition and its place as a domestic utility is firmly established. The relatively high cost of heat as generated through electric energy is in a great measure counterbalanced in the flat-iron by high efficiency in its use. In the electric iron, the heat is developed in the place where it can be used to the greatest advantage, and transmitted to the face of the iron with but very little loss. Because of this direct application the cost of operation is but slightly in excess of the other methods of heating. The electric flat-iron has now become a part of the equipment of every commercial laundry, where electricity can be obtained at a reasonable rate. The popularity of the electric iron is due to its cleanliness and to the increased amount of work that may be accomplished through its use. Because of the time saved in changing irons and the comfort of the room by reason of its lower temperature, a sufficiently greater amount of work is accomplished to more than compensate for the greater cost of heat. The electric current is conducted to the flat-iron from the house circuit by wires made into the form of a flexible cord. The cord attaches to the electric-lamp fixture by a screw-plug and connects with the iron by a special attachment piece as indicated at _P_ and _R_ in Fig. 226. Connection is made to an incandescent lamp socket at any convenient place. The only precaution necessary in attaching the iron is to see that the fuse and the wires, which form the circuit, are of size sufficient to transmit the amount of current the iron is rated to use. As explained later, the fuse which is a part of every electric house circuit, and the conducting wires which form the heater circuit, must be sufficient in size to transmit the necessary current without material heating. [Illustration: FIG. 226.--Electric flat-iron and its attachments.]