It is often desirable to operate experimental apparatus and other electrical devices from the 110 volt lighting circuit. Such a circuit may be either direct or alternating, commonly spoken of as D.C. and A.C. The easiest method of reducing the voltage of the 110 volt circuit to a value where it can be used in place of an ordinary battery is to use a step-down transformer. A transformer will, however, operate only on alternating current and it is impossible to use it on a direct current circuit. [Illustration: FIG. 48.—A Lamp Bank consisting of a Set of 110-Volt Lamps connected Multiple and arranged to be placed in series with any device it is desired to use on the 110-Volt Current.] It is therefore necessary to secure the desired result by placing an adjustable resistance in the circuit and the most practical method of accomplishing this is to use a "lamp bank." *A Lamp Bank* consists of a number of incandescent lamps arranged on a board so that any desired number may be quickly included in or withdrawn from the circuit. Figure 48 illustrates such an arrangement. It consists of a number of Edison sockets of the type known as "flat-base porcelain receptacles", mounted in two rows upon a suitable baseboard. Six or ten receptacles will usually prove sufficient for the ordinary lamp bank. The lamps are connected in "series-multiple." The diagram in the upper left hand corner of Figure 48 shows just how this is done. Four binding posts should be mounted upon one end of the board and marked A, B, B and C as shown. B and B are connected together, or to use an electrical term, are "in common." The 110 volt supply is connected to the posts B and C. If any electrical device is then connected to A and B and a lamp placed in each one of the receptacles, the lamps will be in series with the apparatus and lower the voltage of the 110 volt circuit to a value where it may be used for recharging storage cells, operating toy motors, electroplating, lighting miniature lamps, running toy railways, etc. 110 volt carbon lamps should be used on a lamp bank operated on a 110 volt circuit. Carbon lamps consume more current than tungsten lamps and it will therefore not be necessary to use as many. Each 16 c.p. 110 volt lamp used will allow about one-half of an ampere to pass through the circuit. Thirty-two candle power bulbs of the same voltage will pass about one ampere, while an 8 c.p. bulb will only permit one-quarter of an ampere to flow. It is therefore very easy to regulate the amount of current flowing by using lamps of different sizes and screwing them in or out of the receptacles. A lamp bank such as this may of course be used on either the 110 volt A. C. or D. C. circuit. CHAPTER IV. HOW AN ALTERNATING CURRENT MAY BE CHANGED INTO DIRECT CURRENT BY MEANS OF AN ELECTROLYTIC RECTIFIER.