ON IMPROVEMENTS IN ASTRONOMICAL CLOCKS. 



59 



Fig. 5. 



section and dd' in elevation. The springs are in metallic connection 

 with the platinnm wire e, which is hermetically sealed through the tube 

 A. To the other platinum wire /, the inner end of which is beaten 

 out into a thin sjiring, is attached a carbon point cj, h being the cor- 

 responding carbon plate attached to the pen- 

 dulum, just below the suspension springs. The 

 whole is enclosed in the tube A, which is ex- 

 panded into a bulb at the lower end, exhausted 

 from the end B, and hermetically sealed. 



On placing a source of radiation in front of 

 the blackened surface h, and allowing a screen 

 to move to and fro between the source of radia- 

 tion and the bulb, contact will be alternately 

 made and broken" between the carbons g and li. 

 In order to give an idea of the amount of radia- 

 ting force required to produce a Crookes' pres- 

 sure of sufficient power to work an instrument 

 of this kind, I will mention that a candle placed 

 four or five inches ofi" the bulb, with a concave 

 reflector at the back, answers exceedingly well, 

 providing the surface 6 is about \\ square 

 inch in area. The actual efifective force also 

 depends to a great extent upon the distance 

 between the surface h and the glass envelope. 

 For this reason I have tried using a clear mica 

 screen, placed inside the bulb very close to the 

 black surface ; but although theory would indi- 

 cate the advisability of so doing, practice shows 

 that very little advantage is gained by the in- 

 troduction of such a screen, the fact being par- 

 tially accounted for by its forming a second 

 obstruction to the radiant force from the light 

 used to work the relay. 



By the introduction of carbon contacts I had hoped to have entirely 

 avoided their sticking together when the current passed. Although for 

 all practical purposes their employment together with the pendulum form 

 of instrument has suflBciently reduced this sticking, yet to a certain ex- 

 tent it still remains a drawback to the use of such a delicate force for 

 making contact as that to be obtained from this indirect action of the 

 radiation from a small lamp or candle. 



When the contacts merely pass the current through a short length of 

 straight wire, there is little or no sticking, but on the introduction of 

 an electro-magnet, a bright spark passes between the contacts, and stick- 

 ing occurs. The spark is well known to be due to the discharge of the 

 extra currents set up in the coils of the magnet, and I expected that 

 both the spark and the sticking would disappear on attaching a tin-foil 

 condenser to the terminals of the relay. On trying this experiment 

 the spark was reduced, but there was no observable alteration in the 

 sticking. 



This sticking is probably due either to the carbon containing a 

 fusible ash, or the attraction caused by the close proximity of the two large 

 surfaces of oppositely charged carbon, large compared with the part that 

 absolutely touches and through which only part of the current would be 



