The various other fixtures of Fig. 219 that are designated as reflectors are in some cases only a means of diffusion of light. In the use of the high-efficiency gas-filled lamps the light is too bright to be used directly for ordinary illumination. When these lamps are placed in opal screens of the indirect or the semi-indirect form the light produced for general illumination is very satisfactory. Considerable light is lost in passing through the translucent glass but this is compensated by the use of the high-efficiency lamps and the general satisfaction of light distribution. =Lamp Transformers.=--Lamps of the Mazda type, constructed to work at the usual commercial voltages, are made in low-power forms to consume as little as 10 watts; but owing to the difficulty of arranging a suitable filament for the smaller sizes of lamps, less voltage is required to insure successful operation. The lamps for this purpose are of the type used in connection with batteries and require 1 or more volts to produce the desired illumination. When these little lamps are used on a commercial circuit, the reduction of the voltage is accomplished by small transformers, located in the lamp socket. The operating principle and further use of the transformers will be explained later under doorbell transformers. The lamp transformer, although miniature in design, is constructed as any other of its kind but designed to reduce the usual voltage of the circuit to 6 volts of pressure. The socket is that intended for the use of the Mazda automobile lamp giving 2 candlepower. This lamp used with electricity at the average rate per kilowatt can be burned for 10 hours at less than half a cent. In bedrooms, sickrooms and other places where a small amount of light is necessary but where a considerable quantity is objectionable, the miniature lamp transformer serves an admirable purpose in adapting the voltage of the commercial alternating circuit to that required for lamps of small illuminating power. Such a transformer is shown in Fig. 220. [Illustration: FIG. 220.--Miniature lamp transformer complete and the parts of which it is composed.] The figure shows in _A_ the assembled attachment with the lamp bulb in place. The part _B_, the transformer, changes the line voltage to that of a battery lamp. A line voltage of 110 may be transformed to suit a 6-volt miniature lamp. The parts _C_ and _D_ compose the screw base and the cover, in which is fitted the transformer _B_. =Units of Electrical Measurement.=--The general application of electricity has brought into common use the terms necessary in its measurement and units of quantity by which it is sold. The volt, ampere and ohm are terms that are used to express the conditions of the electric circuit; the watt and the kilowatt are units that are employed in measuring its quantity in commercial usage. The use of these units in actual problems is the most satisfactory method of appreciating their application.