About 18 months ago I put into use in my office the Vetter method of
controlling the strength of the current derived from the Edison 110
volt system of electrical distribution. The controlling devices were
a 16 c.p. lamp and a pulverised carbon rheostat. By these means a
milliampere, or fraction thereof, up to 100 or more, if desirable, can
be administered to a patient (_see_ diagram, Fig. 58, on next page).
On several occasions when the electrodes of the system above described
were permanently attached to some part of a patient’s person and a
spark was being administered to another patient seated upon a platform
charged by an influence machine, some 15 ft. distant in the same
room, the first patient would exclaim and protest against receiving
a considerable shock. On one occasion, when the continuous current
electrode was in the neighbourhood of a patient’s temple, the patient
experienced the sensation of a flash of light; on other occasions
muscular contractions were produced, always simultaneously with the
spark. Also upon the occurrence of the spark and shock the needle of
the milliamperemeter, a vertical one and calibrated to a wide range
of movement over 5 milliamperes, flew across the scale from, for
instance, 2 to 5 milliamperes and remained at the higher reading. That
a spark occurring 15 ft. away should cause a shock to a person in an
independent circuit excited my wonder; it was inexplicable and yet
so certain to occur that I was obliged to abandon the use of the two
pieces of apparatus at the same time.
At last, when time permitted, I set out to investigate. I sought for
an ordinary induction circuit of parallel wiring and found none. I
then suspected the microphonic rheostat of pulverised carbon and
having cut it out of circuit I substituted for it a water rheostat.
The phenomena now failed to occur. Replacing the carbon rheostat and
putting a telephone in circuit I adjusted the milliammeter to read 2
milliamperes, causing an assistant to evoke the distant spark. All
was now clear. At each spark the needle jumped forward and a distinct
telephone click was heard from the telephone receiver. I observed that
the first jump of the needle was the longest as well as the first
click in the receiver the loudest, both needle jump and click, dying
away gradually at each successive spark until they ceased at from the
twentieth to thirtieth. To turn the rheostat off and then on again
rendered the experiment repeatable. The reading of the meter, best
adapted to success, was about 5 milliamperes though 20 to 50 yielded
good results.
[Illustration: FIG. 58.]
Unable to furnish any reason why the electric radiation of a distant
spark should reduce the resistance of pulverised carbon I refrained
from publishing the bare observation in the hopes of finding an
explanation by further experimentation, merely noting to friends the
delicacy of the pulverised carbon rheostat as a detector of Hertzian
waves and making some further experiments with it and a telephone
receiver in circuit in this direction.
The recent publication of the brilliant researches of Dr. Oliver J.
Lodge now makes the entire matter clear. Dr. Lodge describes a new
form of microphonic detector of Hertzian waves, consisting of two or
more pieces of fairly clean metal in light contact and connected to
a voltaic cell, a film of oxide of the metal intervening between the
surfaces, “so that only an insignificant current is allowed to pass.”
He writes: “Now let the slightest surging occur, say, by reason of a
sphere being charged and discharged at a distance of 40 yards; the film
at once breaks down--perhaps not completely, that is a question of
intensity--but permanently.”
This detector, Dr. Lodge terms a “coherer” because of the partial
metallic cohesion above described. Upon this point he says: “A
bad contact was at one time regarded as a simple nuisance.” ...
“Hughes observed its sensitiveness to sound waves, and it became the
microphone. Now it turns out to be sensitive to electric waves, if
it be made of any oxidisable metal (not of carbon) and we have an
instrument which might be called a micro-something but which, as it
appears to act by cohesion, I call at present a coherer.” The cohesive
result between the metallic surfaces is also referred to as a “welding
effect of an electric jerk.” In the volume just published, entitled
“The Work of Hertz and Some of His Successors,” reprinted from _The
Electrician_, London, this foot note is added on p. 30: “FitzGerald
tells me that he has succeeded with carbon also.”
My experiments would seem to fully demonstrate that carbon as well as
metals may act as coherers. At some recent trials the editors of the
_Electrical Engineer_ were present and were fully satisfied as to the
swinging up of the needle of the milliamperemeter and the click in the
telephone receiver, by repeated tests.
The experimental side of the subject has been so exhaustively and
admirably presented by Dr. Lodge (detailed in the publications referred
to) that what is here said has no more than a secondary interest.
But it may not prove amiss to gather together all the evidence which
tends to demonstrate the influence of disruptive discharges upon
neighbouring bad contacts conveying currents. As Lodge points out,
fuses may easily be “blown out” in this manner. This has occurred to me
on a number of occasions with 10 ampere fuses. Under proper conditions
of sparking surfaces and circuit a short spark might suffice.
May it not also be the fact that the fuses melted during thunderstorms
in their neighbourhood are melted by reason of the effect of the
electric radiations or surgings of the lightning stroke throwing a rush
of the current already in the circuit through the fuse rather than
by the addition of any new current to the circuit by the atmospheric
electricity itself. In this connection Lodge writes: “There are some
who think that lightning flashes can do none of these secondary things.
They are mistaken.” In this as in other directions the new facts have
a practical bearing and a pursuit of further experiments may lead, as
often happens, to unexpected developments.
So far as carbon contacts are concerned and the fact that Hertzian
waves, like mechanical motion, reduce their resistance, a curious
problem is suggested as concerns the principle upon which some carbon
transmitters act. An exclusive monopoly of all carbon transmitters is
based upon the claim that the variations in resistance are produced by
variations in _pressure_ due to a mechanical force, viz., sound waves.
If my experiments, above detailed, are exact, two facts appear:
1. That another form of motion, ether vibration, causes a variation of
resistance of carbon contacts.
2. That it remains to be proved that variation of _pressure_ is the
only means of varying the current strength, for variation of molecular
contact occurs in the present instance without any evidence that it is
due to variation of pressure.