April lo, 1879] 



NATURE 



535 



from a hot pole of the bulb will strike the inner surface 

 of the sloping vanes, and driving them before it, will 

 cause a rotation which appears negative to an observer, 

 although it is really positive to the direction of pressure. 

 Fig. II sufficiently illustrates this mode of action. The 

 heat is supposed to be applied near the centre, and the 

 molecular pressure, radiating on all sides, presses the 

 vanes chiefly on the inner surfaces. The anomalous 

 results obtained when the radiometers were heated with 

 hot glass shades or hot water are thus accounted for. 

 Polar heating gives negative, and equatorial positive, 

 rotation, and when both are applied together by immersion 

 in hot water, the direction of motion is governed by the 

 stronger of these two forces. 



In my description of Fig. 7 (p. 534) I showed that the 

 glass heated by the ultra-red rays became hot, and acted 

 on the driving surface, generating molecular pressure, and 

 causing the sloping vanes to turn in the positive direction. 

 At the same time the vanes get warm and become them- 

 selves sources of molecular pressure. The amount of 

 molecular pressure thus generated depends on the capacity 

 of the material of the vanes to absorb heat. Thin mica 

 will hold very little, thick mica will hold more, and 

 aluminium will hold most. This extra capacity for heat 

 causes more molecular pressure to proceed from the 

 aluminium and thick mica, and generates a proportionate 

 amount of driving power on the surfaces of the vanes, 

 turning them in the positive direction, and supplementing 

 the action of the equatorial ring of hot glass. 



The next subject of investigation was the action of radia- 

 tion on cones, cylinders, and cup-shaped vanes. A pair of 



^^ 



thin aluminium disks, cut half across the diameter, were 

 bent into cones and mounted on two arms as a radiometer, 

 the cones facing opposite ways. Several experiments were 

 tried and repeated with cones of different material. The 

 movement which appeared most anomalous was the 

 attraction observed when a candle was allowed to shine 

 on the hollow side of a cone or cup-shaped radio- 

 meter, the light being screened off the retreating side. 

 Further experiments, however, showed that the effect 

 of bending the plates, or of making cones of them, 

 is to produce a more favourable presentation to the 

 inner surface of the glass bulb. Radiation falls from 

 the candle on the aluminium ; some is reflected and lost, 

 but a portion is absorbed, to be converted into thermo- 

 metnc heat or heat of temperature. Aluminium being a 

 good conductor of heat, and the thickness of metal being 

 insignificant, it becomes equally warm throughout, and a 

 layer of molecular disturbance is formed on each surface 

 of the metal. At a low exhaustion the thickness of this 

 layer is not sufficient to reach from the metal cone to 

 the side of the glass bulb ; as the exhaustion increases, 

 this layer extends further from the generating surface, 

 until at a sufficiently high exhaustion the space between 

 the side of the glass bulb and the adjacent portion of the 

 metallic cone is bridged over, and pressure is exerted 

 between the two surfaces. Fig. 12 shows how this pres- 

 sure will act. The direction of pressure is indicated by 

 dotted lines issuing from the metal cone. The more 

 favourable presentation offered by the cone causes the 

 pressure to be greatest between the glass bulb and the 

 outside of the cone ; the pressure from the inside of the 



cone and from the outside, away from the side of the 

 glass, is dissipated without acting, but the pressure 

 between the glass bulb and the side of the cone nearest 

 to it is active ; the cones, therefore, are pressed round in 

 the direction of the arrows, and the motion has the 

 appearance of attraction. 



Cones being inconvenient in shape, I employed por- 

 tions of cylinders wherewith to shape the vanes, and I 

 ultimately found that cups were more easily affected by 

 radiation than portions of cyUnders, whilst they are more 

 easily fashioned. I found that a four-armed cup-shaped 

 aluminium radiometer, the cups being bright and 10 

 millims. in diameter, and the radius of the curvature 

 being 6 millims., rotates in the light as well as a 

 flat vaned instrument. I sealed one of these instru- 

 ments on to the mercury pump. During exhaustion 

 accurate observations were taken of the number of revo- 

 lutions per minute caused by one or more standard 

 candles 3 inches from the centre of the bulb. I also 

 took observations of pressure, and the exhaustion was 

 carried to a very high point. Fig. 13 shows the curve 

 plotted from these observations, taking the rarefaction of 

 the air in millionths of an atmosphere as abscissae, and 

 the number of revolutions a minute as ordinates. The 

 curve traced through the dots representing observations 

 illustrates the gradual increase of sensitiveness up to a 

 certain point of rarefaction, and the sudden drop after 

 that point is reached. 



To still further investigate the action of dark heat on 

 the vanes, I contrived an apparatus to which I could 

 apply a very intense source of heat always ready in the 



same place, the heat not having to pass through glass, 

 and being completely under control as to intensity and 

 time of action. The instrument with which I performed 

 the great number of these experiments is shown in Fig. 14. 

 The cylinder is sealed at the top so as to permit of the 

 highest possible exhaustion. It is dra%vn off narrow at 

 the end, and a stem is sealed in to hold a needle- point. 

 To the narrow end a fine tube is attached to connect the 

 apparatus to the mercury-pump. Round the needle is 

 placed a ring of fine platinum wire, a a, the ends of which 

 are joined to thicker platinum wires passing through the 

 glass. The fly consists of four square vanes of clear 

 mica, b b, inclined at an angle of \i^ to the horizontal 

 plane and supported on light aluminium arms. Above 

 the vanes is a flat disk of clear mica, c c, having a glass 

 cap in its centre, and easily rotating on a needle-point. 

 The vanes and the mica disk are supported independently 

 of each other on separate needle points, which are held in 

 glass rods, d, d, d. A current of electricity from two 

 Grove's cells, turned on or off by a contact key, gives the 

 power of making the wire ring, a a, red hot when desired. 

 The normal or positive movement of the disk is in the 

 opposite direction to that of the vanes ; thus, if the 

 positive movement of the vanes is in the direction of the 

 hands of a watch, the positive movement of the disk is in 

 the opposite direction. With the apparatus fuU of air at 

 the ordinary pressure (bar. = 761 millims.) the direction 

 of rotation, both of the vanes and disk, is positive when 

 the platinum wire is ignited. The speed of the vanes is 

 1 3*3 revolutions a minute, and that of the disk i a 

 minute 



