84 Sir Oliver Lodge [March 24, 



or valvelike action can be exerted on pulses of opposite direction 

 even if they are urged by forces of the same magnitude ; and if the 

 two terminals are put into a vacuous receiver instead of being left in 

 common air, the ease with which they will transmit electricity either 

 in one direction or the other depends upon the thoroughness of the 

 vacuum ; and in certain states of the vacuum the effect may be very 

 marked, as has been known for some time. 



Mr. Edison observed some years ago that if one of the terminals 

 was hot and the other cold, the valve-tendency, or rectifying-action, 

 was singularly perfect and effective at very low forces ; for instance, 

 if the incandescent loop of an ordinary Swan lamp be used as one 

 terminal, and a piece of cold metal in the same bulb be used as the 

 other. Sir William Preece found that such an arrangement could 

 hardly convey a moderately propelled current in one direction, while in 

 the other direction it could easily convey such a current even though 

 driven with infinitesimal force ; and an arrangement Hke this has 

 been recently adopted by Professor Fleming to make excessively 

 feeble alternating currents record themselves on an ordinary sensitive 

 galvanometer, and thus to get an instrument metrically responsive to 

 the faint alternating impulses received at a distant station in wireless 

 telegraphy. 



These devices deal chiefly or wholly with impulses of low tension. 

 For high-tension work the mercury-lamp invented by Mr. Cooper- 

 Hewitt, which excited so much interest a few years ago, when 

 supplied with the proper appendages designed by him, acts in a 

 surprisingly efficient manner. 



It consists of a tube containing nothing but mercury vapour, 

 with electrodes ingeniously arranged so that at a certain stage of 

 exhaustion the current shall be transmitted with fair ease in one 

 direction and barely able to go at all in the other, unless driven with 

 much greater force ; and these lamps are till lately the rectifying 

 device or valve which in the fog experiments I have chiefly em- 

 ployed. They are, however, for some purposes not overportable : 

 the strain on the glass is great, and they are rather liable to break ; 

 they are excellent for the purpose for which they were intended — as 

 lamps — but as rough-and-ready valves for discriminating between 

 opposite rushes of electricity on an extensive scale they leave much 

 to be desired ; so that I now employ a new and specially designed 

 form of valve with special appliances for preventing excessive strain 

 upon the glass. 



Other Uses. 



Other uses than smoke deposition can be found for electric 

 valves in connection with a high-tension intermittent supply ; for 

 instance, they can be used in any portable arrangement for the 

 sending end of wireless telegraphy, as well as for metrical purposes 



