60 EEPORT — 1880. 



passing. I have tried several kinds of carbon for the contacts, but the 

 finest electric-lamp carbon seems to be the only available sort, the resistance 

 of more compact carbons being too high. I have also tried using contacts 

 of platinum, iridium, also one of platinum and the other of gold, platinum 

 and iridium, carbon and iridium, carbon and platinum, all of which stick 

 together more than when both are of carbon. 



In order to overcome the, for the present, inevitable amount of sticking 

 of the carbon contacts, it is necessary to multiply the force for making 

 and breaking contact by means of long leverage. It will be seen that 

 in the pendulum an^angement described, any amount of leverage can be 

 easily obtained without the friction or resistance that would be caused by 

 pivots. 



The force, also, obtainable from a given source of radiation, is greatly 

 augmented in this instrument by the use of a screen placed a little dis- 

 tance behind the blackened surface, but fixed to it as part of the pendulum 

 bob. In this way nearly the maximum amount of Crookes' pressure 

 is obtained, all acting in the one direction, whereas, if there be no screen 

 behind the black surface, the heat transmitted through the blackened 

 mica sets up a considerable Crookes' pressure, which acts between the 

 bulb and the back of the blackened mica, considerably reducing the 

 effective force in front. 



In experimenting with these various radio-relays, I have used a 

 seconds pendulum, having an escapement similar to that described by 

 Mr. Gill in his report for last year. 



It has been necessary to use an ordinary, but very sensitive, relay 

 between the radio-relay and the pfendulum, as it is best to have as weak 

 a current as possible passing through the carbon contacts. 



I regret that my experiments in the radio-relay part of the subject 

 should have extended over such a long period, but the time I have at my 

 disposal for original work is very limited. 



I also regret that I cannot be present at the meeting this year, to 

 show the various relays, and receive suggestions from the members of 

 Section A. I shall, however, carefully study any discussion that may be 

 recorded on the subject, and in the continuation of the experiments make 

 use of any suggestions with great pleasure. 



C. H. GiMINGHAM. 



Dear Mr. Gimingham, — I return you herewith Mr. Gill's letter and 

 diagrams. The principle of his proposed arrangement seems admirable, 

 provided a perfect system of contacts could be devised, and your plan for 

 them is unexceptionable in theory ; but as it appears that the carrying out 

 of the details may be a little troublesome, I have had recourse for the 

 present to a more simple contrivance, which, though not so perfect 

 theoretically, will, I believe, be found to work very well in practice. 



I annex a figure (fig. 6) which represents the arrangement. A very 

 small magnetised needle A a is pivoted as a compass needle on a vertical 

 pin b. In a plane above or below this is pivoted a light forked lever d dd 

 so placed that a pin c in the magnetised needle, hits one or other of the 

 prongs of the fork dd SiS it swings from side to side. At the extreme end 

 of the lever d is fixed a fine fibre of spun glass slightly buckled by the 

 screw t ; this has the effect of putting the forked lever d d into a state of 

 unstable equilibrium and compelling it to keep in contact with one or 

 other of the contact screws s s'. The whole apparatus is enclosed in an 



