226 



NATURE 



\yan. II, 1877 



increased quickly in speed until they revolved at a rate of 

 one in four seconds. This movement kept up for several 

 minutes, and as it slackened it could at any time be 

 revived by a few drops of ether on the bulb. When in 

 rapid movement a hot glass shade was placed over the 

 radiometer the movement slackened, the arms quickly 

 came to rest and ,immediately revolved in the reverse 

 direction, acquiring a speed of about two revolutions a 

 minute, and keeping up this reverse movement for more 

 than ten minutes. 



I again set the instrument in rapid rotation by dropping 

 ether on the top of the bulb and applied the tip of one 

 finger to the side of the bulb for ten seconds. The rota- 

 tion stopped, and I could not start it again for some 

 minutes, although I dropped ether on the bulb, several 

 times in the interval. When the radiometer had once 

 more acquired the temperature of the air I dropped ether 

 on the bulb, not in the centre, but so that the ether wetted 

 only half of the bulb. The arm which was nearest to the 

 part most chilled by the ether rushed towards that part 

 and remained, as it were, fixed opposite to it, refusing to 

 move away, although I tried to equalise the temperature 

 by dropping ether on the other parts of the bulb, and to 

 drive it round by bringing a candle near. Not until the 

 candle came within six inches of the bulb did the arms 

 begin to rotate, which they then did with a rush, as if 

 suddenly relieved from a state of tension. 



I have referred to a sufficient number of experiments 

 to show that a metal radiometer rotates in a negative 

 direction on being exposed to the action of dark heat, 

 the black advancing and continuing to do so until the tem- 

 perature has become uniform throughout. On removing 

 the source of heat, the fly commences to revolve with 

 rapidity the positive way, the black this time retreating as 

 it would if light shone on it. 



To determine whether at temperatures between 250° 

 and 100° the repellant action of radiant heat was about 

 equal on black and on white surfaces, I used a radiometer 

 having pith discs blackened on one side. A tube was 

 sealed into one side of the bulb, and having two stout 

 platinum wires passing along it, sealed their whole length 

 in glass to prevent leakage of air into the interior of the 

 apparatus. At the ends of the wires a spiral of fine pla- 

 tinum wire was fastened, and the other ends terminated 

 in loops outside. The bulb was perfectly exhausted, and 

 the following experiments were tried : — 



A resistance-coil was so adjusted that a battery would 

 keep the platinum spiral at a bright red heat. The arms 

 of the radiometer, which were before quite still, moved 

 rapidly until two of the discs were one on each side of the 

 hot spiral, the black disc being further off than the white 

 disc. The resistance was then gradually increased, and 

 as the temperature of the spiral diminished, the black 

 disc gradually approached the spiral, until, when the tem- 

 perature was just at the point of visible redness in a dark 

 room, the black and white discs were practically equidis- 

 tant from the spiral. On diminishing the resistance, the 

 same phenomena took place in inverse order. 



The resistance was again adjusted to give a bright red 

 spiral, and the contact key kept pressed down. A hghted 

 match was momentarily brought near the bulb, so as to 

 start a movement. Rotation of the arms commenced, 

 and kept up, with some energy, at the rate of about one 

 revolution in five seconds, equal to that given by a candle 

 eight inches off. There was some little hesitation, as the 

 white side came up to the spiral, but this was scarcely 

 noticed when the speed had become steady. The resist- 

 ance was now slightly increased. The speed became 

 slower as the temperature of the spiral diminished, and 

 the hesitation, as the white approached the spiral, became 

 more apparent. The resistance was further increased, 

 with the effect of making rotation still slower, bring- 

 ing the temperature of the spiral down to just visible 

 redness in the dark. The speed of rotation again 



slackened ; at each approach of the white surface to the 

 spiral it appeared to stop, hesitate, and then get past with 

 a rush. Thus it went on for a few revolutions, until one 

 white disc, a little nearer, perhaps, than the others, was 

 not able to pass, and the arms, after a few oscillations, 

 came to rest, the black and the white surfaces being, as 

 near as I could judge, equidistant from the hot spiral. 



I now tried to ascertain whether, at temperatures lower 

 than 100° C, the white would be repelled most. 



The resistance of the coil was increased again, and the 

 position of the arms in respect to the spiral noticed. 

 When so much resistance was offered to the passage of 

 the current that the spiral would only be just warm, I 

 fancied the white was further from it than the black, but 

 the observation was not satisfactory at higher tempera- 

 tures ; up to visible redness the repulsion was equal for 

 each. Breathing on the bulb sent the arms rapidly round 

 the reverse way. 



The battery was disconnected from the instrument, and 

 one end of a wire was attached to one of the platinum 

 loops, the other end of the wire being connected to the prime 

 conductor of a frictional electrical machine. A few turns 

 of the handle sent the arms flying about wildly, first in the 

 positive and then in the negative direction, till finally 

 one pointed steadily to the platinum spiral, and refused to 

 move. When the candle was quite close it overcame the 

 interference, and the discs revolved in an irregular jerky 

 manner. In three or four days the electrical disturbance 

 was sufficiently diminished to enable me to proceed with 

 my experiments, but I could detect the influence for 

 weeks after. 



One pole of a small induction-coil capable of giving 

 half-inch sparks in air, was fastened to the platinum 

 loops, the other pole being held by an insulating handle. 

 The loose pole was then brought near the bulb. The 

 nearest disc rushed round to it and followed it a little, 

 then it stuck as if the glass were electrified. By gently 

 moving the loose pole round I could get the arms to 

 rotate in either direction, and they would keep on for five 

 minutes or more when once started. These movements 

 appear all to be explained by the known laws of static 

 electricity, the rotations being of the " electrical fly " 

 kind. 



I obtained rotation in a radiometer without having the 

 surfaces of the discs differently colourc;d. One having 

 the pith discs lamp-blacked on both sides, and weighing 

 I "25 grain, was exhausted with a charcoal tube attached. 

 On a candle being brought near it, the arms moved until 

 two of the discs were equidistant from the flame, and no 

 amount of initial impulse in either direction would set it 

 in rotation. A piece of ice caused it to move until one 

 disc pointed to the ice, when it also stopped, but by 

 shading the candle with a screen, so that the light shone 

 on only one half of the tube, rapid rotation commenced, 

 which, by altering the position of the screen to the other 

 side, was instantly stopped, and changed into as rapid 

 rotation in the opposite direction. 



To enable me to exhibit the movement of a radiometer 

 to a large audience I have made an instrument, the discs ot 

 which are of thin glass, silvered and polished on one side, 

 and coated with lampblack on the other. Owing to its 

 great weight the movement is somewhat slow, but in 

 the sun, or, with a strong light shining on the instrument, 

 it is very striking, as it shows discs of light chasing each 

 other round the room. 



To communicate motion from the interior of the bulb 

 to the outside, a radiometer was made which would 

 carry round a magnet. Outside the bulb of this instru- 

 ment I suspended, in a vertical position, a smaller magnet 

 having the south pole at the top and the north pole at the 

 bottom ; this oscillates to and fro with every revolution of 

 the radiometer, and making contact at the bottom, carries 

 an electric current from a battery to a Morse instrument 

 through which a ribbon of paper is drawn by clockwork 



I 



