14 
shall explain; the wheels and rails will serve as the 
return wire. This system worked well at the workshop. 
In practice a special difficulty was encountered. The 
dirt sticking to the rails and felloes of the wheels formed 
a sort of crust so insulating as to prevent adequate com- 
munication with the earth. The increase of resistance 
produced by this interposition of finely conducting bodies 
was often sufficient to arrest the vehicle. The remedy 
was happily beside the evil, and a second conductor was 
established parallel with the first, in communication 
with the second pole of the generator, on which runs a 
second traverser, identical with the former. These two 
cars follow on their respective tubes the movements of 
the vehicle, and ensure a good and constant communica- 
tion between the electrical generator and the motor. 
Fig. I represents the carriage and the station at the 
Place de la Concorde. At the height of the knife-board 
are seen the two conducting tubes supported at certain 
distances by posts, and in the intervals by iron wires, 
like the floor of a suspension bridge. The carriage is 
exactly the same as the ordinary tramway car. The 
motor is placed underneath the feet of the inside pas- 
sengers ; itis a Siemens dynamo-electric machine, with 
horizontal inductors similar to that which produces the 
current in the Palais del’Industrie. The distance traversed 
is about 500 metres, and is accomplished in one minute. 
The work expended reaches 8 horse-power in the curved 
Fic. 2.—Traversers conducting the current to the carriage. 
part; on a level straight run it does not exceed 3} horse- | 
The transmission of motion to the wheels is | 
power. 
effected by means of a fall-chain. By a happy coinci- 
dence, which belongs to the very nature of the electric | 
motor, the static effort is maximum when the motor is in 
repose. 
culty is met with from this point of view. 
speed, resistances are introduced into the general circuit, 
which reduces the intensity of the current, and conse- 
quently the work of the motor ; this operation is very 
simply effected by means of a lever placed at each end of 
the carriage. For stopping, the current is broken, and at 
the same time an ordinary brake is applied. 
As to the mode of communication of the conductors 
with the carriage, we have said that it is effected by 
means of two identical traversers ; it will suffice to describe 
one of them. Fig. 2 represents in detail one of these 
traversers. It is composed of a rectangular frame, bearing 
in its centre a wheel, of which the groove R is semi- 
cylindrical, and is applied against the exterior part of the 
conductor C, formed of a brass tube 22 millimetres in 
diameter and slit on its lower part along all its length 
to a breadth of about 1 millimetre. In this tube 
slides a cylindrical core of 12 centimetres in length, on 
which are fixed, at its extremities, two vertical shafts, 
A,B, which support the wheel or roller. Two springs sup- 
ported on these vertical shafts press the wheel against the 
This renders the starting very easy, and no diffi- | 
To regulate the | 
NATURE 
Nov. 35 1881 
| tube, and maintain an elastic contact between the tube 
and the wheel. The carriage may then be moved; the 
wheel runs against the tube, the core glides in the interior, 
without the communication ceasing to be, if not perfect, 
at least quite sufficient for the purpose. Only at times 
a few sparks are seen at the moment when the carriage 
passes the coupling of the tubes ; these sparks are due 
to small instantaneous ruptures of the current which do 
not affect the regular working of the system. The expe- 
riment shows that the wear and tear scarcely affects the 
tube, and bears almost entirely on the core placed in the 
tube ; but nothing is easier than to replace a core. The 
current reaches the machine by the copper conductor F. 
The traction of the carriage is effected by the cords D or 
D’, according to the direction. 
The electric railway of the Palais de l’Industrie pre- 
sents the first practical solution of an electric traction in 
the case of a tramway. Of course it is easy to see how 
this application of electricity is capable of the greatest 
development, and that by modification of details the 
principle might be applied to railways. 
THE BOLOMETER 
N instrument a thousand times more sensitive to 
radiant heat than the thermopile, and capable of indi- 
cating a change of temperature as minute as I-100,000th 
of a single Centigrade degree, deserves the attention of 
the physicist. When to these qualifications it can be 
added that the new instrument is far more prompt in its 
action, and more reliable than the thermopile for the 
quantitative measure of radiation, then, indeed, no apology 
is needed for a detailed description. The instrument is 
termed by its discoverer, Prof. S. P. Langley, the Jo/o- 
meter, or actinic balance. The earliest design of the in- 
FIG.E. 
ventor was to have two strips of thin metal, virtually 
forming arms of a Wheatstone’s bridge, placed side by 
side in as nearly as possible identical conditions as to 
environment, one only of them being exposed to radiation. 
Such radiation would slightly warm the strip and therefore 
alter its electric resistance, and the amount of this change 
would be indicated by the movement of the needle of the 
galvanometer placed in the middle circuit of the “bridge.” 
For various reasons iron was eventually chosen as the 
material for the thin strips, as it combines the qualities 
of tenacity and laminability, with a greater sensitiveness 
in its electric resistance to temperature changes than 
either gold, platinum, or silver. Preliminary experiments 
made with a simple strip of iron in comparison with 
several delicate thermopiles showed the advantage of the 
new method of investigation. A large Elliott thermopile 
of sixty-three pairs, a very sensitive thermopile of sixteen 
small pairs, and a delicate linear thermopile of seven pairs 
of elements were selected. The iron strip taken was 
7 millims. long, 177 millims. broad, and o’004 millims 
