or Roche dry cell. This cell possesses remarkable recuperative powers and low internal resistance. Made in many sizes, the best suited for medical coils is No. 2; dimensions, 5-7∕8 × 2-7∕16 inches. For heavier work the No. 5, 6 × 2-9∕16 inches, and known as the Navy Standard, is recommended. A convenient size for portable medical coils is No. 3, 3¾ × 1⅞ inches, taking up very little room, yet giving a large output. Two of these latter cells enclosed in the coil case will give with a suitably wound primary (No. 18 to 20 B & S) as strong a current as can be used in electrotherapy. For Ruhmkorff coils cells Nos. 6 and 7 (6 × 3 inches and 7 × 3 inches) furnish a most desirable battery for all work not needing the constant operation of the contact breaker, such as wireless telegraphy, gas-lighting, etc. They will do service on X-ray work, but the writer prefers a storage cell or the copper oxide types. The E. M. F. of the above cells is one and six-tenths volts, and current from 9 amperes to the No. 7 size, which gives 24 amperes on short circuit.
As a matter of practice, there is no really dry cell; all so-called cells contain liquid held in suspension, and their output is limited to the amount of fluid. One of this type can easily be made in the following manner: A containing jar is made up of first-quality sheet zinc, the edges being joined by a turned seam and then soldered, the bottom of zinc being also soldered in. In soldering here, as actually in all such operations, be _absolutely sure_ the edges of the metal are clean. The jar is partially filled with the following composition: Oxide of zinc, 1 part; sal ammoniac, 1 part; plaster of paris, 3 parts; chloride of zinc, 1 part; water, 2 parts, all by weight; or sal ammoniac, 1 part; chloride of calcium, 5 parts; calcined magnesia, 5 parts; water, 2 parts, or enough water to make a thin paste. A brass binding post is soldered to the zinc case and a carbon plate having a binding post is inserted in the centre of the cell, care being taken that it does not touch the zinc. A small disc of wood laid in the bottom of the cell will prevent contact at the bottom. Molten pitch or a composition of pitch and rosin in the proportion of 6 to 1 is poured on top, so as to seal the cell. As gas is generated in the cell, a safety valve should be provided, either a piece of porous cane or a short length of hard rubber tube, inside of which have been placed a few strands of woollen thread. This class of cell is so cheap and so many forms are available for choice that it is rarely desirable to make one's own. They will not do for steady current, but only for intermittent work. The large sizes being of low internal resistance, can be used for signalling in wireless telegraphy, where it is not possible to use wet (or free fluid) cells. The principal dry cells on the market are the Mesco, the O. K., the Nungesser, and the Samson semi-dry cell.