890 
ether. It is propagated in straight lines 
through space. It falls on Jupiter, Venus, 
the Earth and every other planet met with 
in its course, and any machine, human or 
mechanical, capable of responding to its 
undulations indicates its presence. Thus 
the eye supplies the sensation of light, the 
skin is sensitive to heat, the galvanometer 
indicates electricity, the magnetometer in- 
dicates disturbances in the earth’s magnetic 
field. One of the greatest scientific achieve- 
ments of our generation is the magnificent 
generalization of Clerk-Maxwell that all 
these disturbances are of precisely the same 
kind, and that they differ only in degree. 
Light is an electromagnetic phenomenon, 
and electricity in its progress through space 
follows the laws of optics. Hertz proved 
this experimentally, and few of us who 
heard it will forget the admirable lecture 
on ‘The Work of Hertz’ given in this hall 
by Professor Oliver Lodge three years ago.* 
By the kindness of Professor Silvanus 
Thompson, I am able to illustrate wave 
transmission by a very beautiful apparatus 
devised by him. At one end we have the 
transmitter or oscillator, which is a heavy 
suspended mass to which a blow or impulse 
is given, and which, in consequence, vi- 
brates a given number of times per minute. 
At the other end is the recewer, or resonator, 
timed to vibrate to the same period. Con- 
necting the two together is a row of leaden 
balls suspended so that each ball gives a 
portion of its energy at each oscillation to 
the next in the series. Hach ball vibrates 
at right angles to or athwart the line of 
propagation of the wave, and as they vi- 
brate in different phases you will see that a 
wave is transmitted from the transmitter to 
the receiver. The receiver takes up these 
vibrations and responds in sympathy with 
the transmitter. Here we have a visible 
* This is published in an enlarged and useful form 
by The Electrician Printing and Publishing Com- 
pany.—W. H. P. 
SCIENCE. 
[N.S. Von. VI. No. 155. 
illustration of that which is absolutely in- 
visible. The wave you see differs from a 
wave of light or of electricity only in its 
length or in its frequency. Electric waves 
vary from units per second in long sub- 
marine cables to millions per second when 
excited by Hertz’s method. Light waves 
vary per second between 400 billions in the 
red to 800 billions in the violet, and electric 
waves differ from them in no other respect. 
They are reflected, refracted and polarized; 
they are subject to interference, and they 
move through the ether in straight lines 
with the same velocity, viz., 186,400 miles 
per second—a number easily recalled when 
we remember that it was in the year 1864 
that Maxwell made his famous discovery of 
the identity of light and electric waves. 
Electric waves, however, differ from light 
waves in this, that we have also to regard 
the direction at right angles to the line of 
propagation of the wave. The model gives 
an illustration of that which happens along 
a line of electric force; the other line of mo- 
tion I speak of is a circle around the point 
of disturbance, and these lines are called 
lines of magnetic force.* The animal eye is 
tuned to one series of waves; the ‘ electric. 
eye,’ as Lord Kelvin called Hertz’s reso- 
nator, to another. If electric waves could 
be reduced in length to the forty-thousandth 
of an inch we should see them as colors. 
One more definition, and our ground is 
cleared. When electricity is found stored 
up in a potential state in the molecules of a 
dielectric like air, glass or gutta-percha the 
molecules are strained ; it is called a charge, 
and it establishes in its neighborhood an 
electric field. When it is active, or in its 
kinetic state in a circuit, it is called a cur- 
rent. It is found in both states, kinetic and 
potential, when a current is maintained in 
a conductor. The surrounding neighbor- 
hood is then found in a state of stress, form- 
ing what is called a magnetic field. 
*Vide Fig. 4. 
