Nov. 30, 1876] 



NATURE 



lOI 



RESEARCHES ON THE RADIOMETER 

 By Prof. Paul Volpicelli. 



I. A LL radiometers do not possess the same sensibility 

 ■^^ necessary for every experiment. 



2. The most sensitive of the two which are in the 

 physical museum of the Roman University shows that 

 the freezing mixture of chloride of sodium and snow, 

 applied to the upper hemisphere of the small globe, pro- 

 duces a rotation of the mill in the same direction in which 

 it is produced by heat radiation, i.e., with the white face 

 of the small discs in advance. 



3. If to this lowering of temperature be added 

 a radiation of heat, the rotation of the appa- 

 ratus is accelerated at the same time. 



4. If the freezing mixture referred to be placed 

 on the lower hemisphere of the same small 

 globe, the apparatus will rotate with the ab- 

 sorbing, i.e.^ the black faces in advance, and 

 consequently in the direction contrary to that 

 of the preceding experiment, i.e., to the direction 

 produced, if to the same lower hemisphere, 

 radiant heat be applied. 



5. If during the rotation produced by the 

 application of the freezing mixture to the lower 

 hemisphere of the small globe we cause radiant 

 heat to strike the same globe, the apparatus will 

 be brought to a stop ; and as soon as the source 

 of heat is withdrawn, the rotation will imme- 

 diately commence. 



6. If the small globe is plunged entirely in a 

 heated liquid, or even in a freezing mixture, 

 the apparatus will remain at rest. 



7. It should be noted that the freezing mix- 7 

 ture applied to the upper hemisphere of the 

 small globe, produces a rotation in the direction 

 opposite to that produced by the same mixture 

 when applied to the lower hemisphere. 



8. It has been stated that the radiometer in 

 darkness remains at rest ; but it should be re- 

 marked that if even in darkness it is affected by 

 dark radiant heat, the apparatus will assume a 

 lotatory movement ; yet the instrument may 

 remain at rest even when placed in a dark 

 space. 



9. The luminous rays of the full moon focussed 

 by means of a lens, do not cause rotation of the 

 instrument 



10. If the radiation of the flame of a Locatelli 

 lamp is caused to traverse several plates of per- 

 fectly transparent glass, it will be seen by the 

 number of turns of the instrument, that the law 

 of De la Roche is verified regarding the ab- 

 sorption of radiant heat through these plates, 

 however many they may be. I have been able 

 by this means to diminish the radiant heat to 

 such an extent as to cause the rotation of the 

 -adiometer to cease, although the light of the 

 same radiation was increased by means of a lens. 



11. The same radiation, that, viz., produced 

 jy Locatelli's lamp, by traversing a saturated *" 



but perfectly transparent solution of alum, before 

 reaching the radiometer, did not set it in motion, although 

 the radiant light was but little diminished ; and the same 

 is the case when the light is increased. 



12. It would appear at present that the rotation of the 

 radiometer depends on radiant heat and not on the 

 lumin( us rays. 



13. It appears also that the mechanical cause of the 

 rotation of the radiometer consists in the motion of the 

 molecules of very rarefied gas in the small globe, which 

 is in accordance with the opinion of modern thermody- 

 namics. 



THE SIPHON RECORDER AND AUTOMATIC 

 CURB SENDER 



"C^OR some time after the introduction of submarine 

 -*- telegraphy Sir William Thomson's mirror galva- 

 nometer was the only instrument delicate enough to re- 

 ceive the signals transmitted through a long cable. The 

 spot of light reflected from the mirror moves over the 

 scale and indicates every change of current in the cable. 

 The clerks by degrees learn to interpret the motions of the 

 spot of light, and are able to read the signals sent. The 

 signals, however, must be read at the instant of arrival, 

 and the clerk has no way of correcting what he receives 



z: 



r 



Fig. I. 



except by having the signals repeated from the distant 

 end. 



The Siphon Recorder was devised, more recently by 

 Sir William Thomson, for the purpose of receiving and 

 recording the signals transmitted through a submarine 

 cable ; though it mav also be used for receiving sig- 

 nals sent along a land line. It actually draws on paper 

 the curves corresponding to the changes of current which 

 pass through the line. Thus a permanent record is made 

 of every signal that is sc-nt, and not only can the clerk be 

 sure that he reads the signals correctly, but also any 



