90 
NATO 
| NovEMBER 22, 1906 
water up into the interior of the hanger—thus keeping the 
bolt dry—and “‘ neither rust nor electrolysis can corrupt.” 
(b) A different form of hanger—simply a metallic link 
between the ear and span wire, and insulated by two or 
three independent external insulators. 
(c) The hanger to be composed of glazed porcelain with 
a plain metal bolt passing through, but the porcelain must 
be kept dry and sheltered from rain. 
Several other points of interest are touched upon by 
the authors, and the discussion which followed the read- 
ing of the paper by Mr. Tweedy proved that the opinions 
on the points raised by the paper were very varied, and led 
to a keen criticism. The idea of a shield was generally 
welcomed, and a suggestion was made that it should be 
manufactured in such a form as to be readily adjusted 
to existing hangers, without having to dismantle the same. 
On the subject of the strength of poles, however, the 
majority was against any reduction in size, and the ques- 
tion of the Standard Committee’s “‘ standard pole ’’ pro- 
voked an animated discussion. 
The subject of the paper is one which for a long time 
has needed discussion, and the interest in it was shown 
by the fact that, after the paper was read and discussed 
at the Birmingham local section’s meeting, it was re- 
discussed in London later in the session, and we may hope 
that the many points and facts brought forward will help 
to mitigate the present existing difficulties of the over- 
head system, and at the same time help to reduce the 
capital expenditure on tramway schemes that may be 
undertaken by local authorities. 
SOME ASTRONOMICAL ‘CONSEQUENCES OF 
THE PRESSURE OF LIGHT3 
(S28 a year ago Prof. Nichols gave here an account of 
the beautiful experiment carried out by himself and 
Prof. Hull which, with the similar experiment of 
Lebedew, proved conclusively that a beam of light presses 
against any surface upon which it falls. Not only did 
Nichols and Hull detect the pressure, which is difficult 
enough, so minute is it, but they measured it with extra- 
ordinary accuracy, and confirmed fully Maxwell’s calcula- 
tion that the pressure on 1 sq. cm. is equal to the energy 
in 1 cubic centimetre of the beam. 
Thus we have a new force to be reckoned with. It is 
apparently of negligible account in terrestrial affairs, partly 
in that it never has free and uninterrupted play. But out 
in the solar system, where there is no disturbing atmo- 
sphere, and where it may act without interruption for 
ages, it may produce very considerable results. Even here, 
so minute is the force that it need only be taken into 
account with minute bodies. Prof. Nichols in his discourse 
told how it may possibly account for the formation of 
comets’ tails if these tails are outbursts of finest dust. 
To-night I shall try to show how it may be of importance 
with bodies which, though still minute, are yet far larger 
than the particles dealt with by Prof. Nichols. Such small 
bodies appear to abound in our system, and to reveal their 
existence on any starlight night when perishing as shoot- 
ing stars. 
We are to examine, then, how the pressure of light, or 
more generally the pressure of radiation, from one end of 
the infra-red to the other end of the ultra-violet spectrum 
will affect the motion of these small bodies. 
I think we get a clearer idea of the effects of light or 
radiation pressure if we realise from the beginning that 
a beam of light is a carrier of momentum, that it bears 
with it a forward push ready to be imparted to any surface 
which it meets. 
Thus, let a source a (Fig. 1) send out a beam to a 
surface B, and to bring out this idea of carriage of 
momentum let A only send out light for a short time, so 
that the beam does not fill the whole space from a to Bs, 
but only the length cp. While the beam is between a and 
B, B feels nothing. But as soon as p reaches B, B begins 
to be pushed, or it receives momentum in the direction 
AB, and will continue to feel the push or receive momentum 
until c has reached B, when the push will cease. The 
1 Discourse delivered at the Royal Institution on May 11, by Prof. J. q. 
Poynting, F.R.S. 
NO. 1934, VOL. 75 | 
existence of this push on B is definitely proved by the 
experiments of Lebedew and Nichols and Hull. Now, 
unless we are prepared to abandon the conservation of 
momentum, this momentum must have existed in the beam 
cp and have been carried with it, and it must have been 
put into the beam by a while it was sending forth the 
waves. A, then, was pouring out forward momentum, and 
was feeling a back push while it was radiating. This 
back push against the source has not, I think, been proved 
to exist by direct experiment, though an indirect proof may 
perhaps be afforded by the case of reflection. When a 
Re ae B 
¢ D 
Fic. 1. 
A 
beam is totally reflected, the push measured in light- 
pressure experiments is double that when it is absorbed, 
that is, there is a push by the incident beam and an equal 
push by the reflected beam, and we may perhaps regard 
the reflected beam as starting from the reflector as source, 
and then we have a push back against the source. But 
whether this be proof or not, I do not see how there can 
be the slightest doubt that the pressure against the source 
exists, and that for the same intensity of beam it is equal 
to that against a receiving surface. 
Some experiments which have been made by Dr. Barlow 
N A 
Sos 
Fic. 2. 
and myself appear to bring to the front this conception of 
light as a momentum carrier. When a beam falls on a 
black surface it is absorbed—extinguished—and its 
momentum is given up to the surface. In a beam of light 
AB (Fig. 2) the momentum is a push forward in the direc- 
tion AaB, and if it falls on a black surface s it gives up 
this momentum to s._ The total push which is in the direc- 
tion AB may be resolved into a normal push n and a 
tangential push tr. If s can move freely in its own plane, 
and only in that plane, T alone comes into play, and s will 
slide towards s. 
B 
A 
Fic. 3. 
end 
the 
To show this effect we fixed two glass discs at the 
of a short torsion rod hung by a fine quartz fibre, 
discs being perpendicular to the rod, and the face of one 
of them being blackened. Fig. 3 shows a plan of the 
arrangement. The apparatus was enclosed in a glazed 
case, which was exhausted to about 2 cm. pressure of 
mercury. On directing a horizontal beam ap at 45° on 
to the black surface B, the normal force merely pressed 
B back, but the tangential force turned B round the point 
of suspension c away from as. It is difficult to make the 
disc quite symmetrical and the beam quite uniform, and 
