NOVEMBER 12, 1903] 
NA TORE 
33 
electron theory of electric action rests; its experimental 
verification is vital to the theory, and, indeed, to the 
whole of the recent views, of the origin of electric and 
magnetic forces. 
A method of testing the existence of the effect is 
given by Maxwell in his “‘ Electricity and Magnetism,”’ 
and was first put into practice, with some modifi- 
cations, by Rowland in 1876. The experiment, carried 
out in Helmholtz’s laboratory, proved to the satisfac- 
tion of Helmholtz and of Maxwell that the effect 
existed; this consisted in rotating a charged disc of 
gilded ebonite near an astatic magnetic needle and 
observing the deflection. This was very small, from 
5 to 7-5 mm., but it agreed with the amount expected 
from the theory. In 1883 Lecher attempted to repeat 
these experiments, but with negative results; Rontgen, 
however, in a research having another object, obtained 
the same effect from a moving charge as Rowland had 
done. In 1889 Rowland and Hutchinson tool the 
matter up, and, modifying the apparatus, obtained 
results which appeared to establish the convection of 
an electric charge on a moving conductor without a 
doubt. 
In 1897 Crémieu began his experiments. If a 
charged moving conductor produces a magnetic field, 
a charged conductor at rest in a varying magnetic 
field should be subject to mechanical force. Crémieu 
failed to detect this force in an experiment arranged 
for its measurement, though, according to calculation, 
it ought, if existing, to have been easily measurable. 
He then attempted Rowland’s reverse experiment, the 
detection of the magnetic field, but modified entirely 
and in an ingenious manner Rowland’s arrangement. 
Imagine a coil placed in close proximity to the rotating 
disc, the planes of the two being parallel, and let 
the ends of the coil be connected to a galvanometer. 
On charging the disc a magnetic field is produced 
near the coil, if the Faraday-Maxwell views be true; 
thus a current is induced in the coil, and a throw 
of the. needle of the galvanometer is the result. 
Then by arranging to charge and discharge the 
disc alternately, and by means of a commutator 
to reverse the galvanometer connections suitably, 
the throw becomes a _ permanent deflection the 
amount of which can be calculated. These experi- 
ments Crémieu carried out with consummate care, and 
the result was negative. The needle remained at rest; 
there was no such thing as electrical convection. 
This fact he verified apparently by several ingenious 
modifications of his apparatus and his method, always 
with the same result; and when at the Glasgow meet- 
ing of the British Association he gave an account of 
his work, probably no one in the room except himself 
accepted his results, but no one, and the critics were 
both acute and numerous, could find the flaw. 
Meanwhile, Rowland had returned to the defence of 
his position; it was almost the last piece of work he 
undertook, and just at the time of Crémieu’s visit to 
Glasgow, September, 1901, there appeared a full 
account of experiments in the Johns Hopkins Labor- 
atory by Pender, who, adopting Crémieu’s induction 
method, again verified Rowland’s result. From the 
observations a value can be found for the velocity of 
light, and Pender found it to be 3.05 x 10!° cm. per 
second, a value sufficiently near to the truth to 
establish beyond a doubt the theory of the measure- 
ments. This was verified by further work published 
early this year; meanwhile, Crémieu continued to 
obtain his negative results. 
Such was the position when Pender was invited to 
Paris to repeat, in the splendid laboratories of the 
Sorbonne, and in conjunction with Crémieu, the ex- 
periments he had made in Baltimore. Funds were 
provided, in part by the Carnegie Institution, in part 
by the Institute of France, and the two experimenters | 
NO. 1776, VOL. 69] 
set to work together, and with the same result— 
Pender verified the Faraday-Maxwell theory, Crémieu 
disproved it. 
Isxperiments were then made on the direct magnetic 
effect, repeating, but with some modifications to meet 
objections of Crémieu, Rowland’s original experi- 
ment. These, again, led to the result that a charged 
surface moving in its own plane produces a magnetic 
field; the very sensitive and permanent astatic system 
employed in this experiment will be found useful else- 
where, but reference must be made to the original 
paper for an account of it. Finally, Crémieu was con- 
vinced; it remained only to account for his negative 
results. : 
It was observed that in all cases he had covered the 
charged conducting surfaces with a thin layer of some 
dielectric other than air, usually india-rubber or mica, 
and there seemed some reason to suppose his failure 
was due to this, and so it was proved to be; the re- 
moval of the dielectric coating from Crémieu’s 
apparatus enabled him to observe the convection effect, 
while by coating the discs used in Pender’s induction 
experiment with mica, the convection effect was re- 
duced by some go per cent. 
How the dielectric acts is still a mystery; it is satis- 
factory, however, that the two experimenters are in 
agreement. The thanks of physicists are due to those 
who suggested and rendered possible this somewhat 
novel collaboration; it is satisfactory to the great 
French physicists who have aided Crémieu with advice 
and assistance that the matter should be settled at 
Paris; it is satisfactory to Pender that he has estab- 
lished conclusively and finally the result which was the 
beginning of Rowland’s brilliant fame. 
At the same time, the question, What does the 
dielectric do? remains an interesting one, especially 
as Vasilesco-Karpen, in a paper which follows the one 
we have been discussing, finds that it has no effect 
on the result. He modified Crémieu’s arrangement 
by introducing a condenser into his circuit of coil, 
commutator and galvanometer, in which induced 
currents are set up by the alternating charge and 
discharge of the rotating discs. By suitably arranging 
the period of this circuit he was able to intensify 
the efforts considerably, and obtained a reasonably 
satisfactory agreement between his results and theory. 
The disc, as was the case in most of Pender and 
Crémieu’s experiments, moved between condensing 
plates, and Vasilesco-Karpen made the following four 
series of experiments:—(1) Disc bare, condensing 
plates bare; (2) disc bare, condensing plates covered 
with glass 4 mm. in thickness; (3) disc covered with 
a thin layer of caoutchouc, condensing plates bare; 
(4) dise as in (3), condensing plates as in (2). 
He states as the result that, so long as the speed 
and the charging voltage are kept constant, the mag- 
netic effect is the same in all these cases. This result 
is opposed to that of Crémieu and Pender, who found 
that coating the disc with a thin layer of rubber de- 
stroyed the convection effect, and so the matter rests 
at present. The magnetic effect due to the motion of 
an electron is confirmed; some of its secondary con- 
sequences remain obscure. ( 
NOTES. 
Tue list of birthday honours includes the following 
names of men known in the scientific world :—Prof. C. Le 
Neve Foster, F.R.S., has received the honour of knight- 
hood. Colonel D. A. Johnston, director-general of the 
Ordnance Survey, has been appointed a Companion of the 
Order of the Bath. Dr. Robert Bell, F.R.S., acting 
director of the Geological Survey of Canada, has been 
appointed a Companion of the Imperial Service Order. The 
