of a glass jar coated on the inner and outer surface with tinfoil about half-way up. The uncoated part of the jar is varnished to minimise loss of charge along the surface of the glass. An improved form of Leyden jar has been designed by Mr. Mosicki, and is largely used in wireless telegraphy. The coating of tinfoil is replaced by silvering, and the shape of the glass vessel is designed with special reference to its ability to withstand very high e.m.f.'s. Whereas the ordinary jar of the physical laboratory can only be used with an e.m.f. of about 20,000 volts, the Mosicki condenser, as made for wireless telegraph stations, can be used up to an e.m.f. of 60,000 volts. Mosicki condensers are also used for the protection of electric power lines from atmospheric electricity, and from the effects of sudden electric disturbances. They act as a kind of electric buffer or elastic link, able to soften the blow which the line and machinery might otherwise receive with full force if there were, from any cause, a sudden increase in the charge on the system.
When the condenser is not subjected to a very high potential difference, the insulator separating its two surfaces or coatings need not be glass, but may be a cheaper material,
