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1932 article on singing arc




-
From: 	William Noble[SMTP:William_B_Noble-at-msn-dot-com]
Sent: 	Friday, June 20, 1997 8:47 PM
To: 	Tesla List
Subject: 	1932 article on singing arc

Norman stanley sent me a photocopy of subject article, but I don't have an E 
mail address to return the OCR'd file to - so I am posting the text portion of 
the document here.  I have the whole document as a word 97 file, so if you 
want it with the pictures, drop me a note and I'll send it out.   

=========================================
Experiments with "Singing
and Tesla Coil

By W. MOELLER (Germany)

SINCE radio discovery y began the methods used to generate high fre- have 
often changed. There was first Hertz's   resonant circuit excited by sparks; 
this method, while applicable to telegraphy creates only damped waves 
unsuitable for modulation In 1900 while seeking a method of created  undamped  
waves, Duddell\ discovered  the "singing arc" he demonstrated that an arc 
light is capable of exciting continuous oscillation in a parallel  resonant 
circuit. In 1906 Valdemar  Poulsen constructed the  first practical arc 
transmitter for transmissionwork; but it was soon superseded by the 
vacuum-tube transmitter.

However, students of  radio development will find experiments with the 
arc-light generator  quite interesting; and they can be performed with com- 
simple means, such as a small laboratory  affords The first is Duddell's 
classic experiment, showing  the generation of alternating current in any 
circuit containing capacity and inductance as soon as it is connected across 
an electric arc

An apparatus like that illustrated in the center, below and corresponding to 
the schematic circuit of Fig. 1, i:; set up. The direct-current supply should 
be about 200 volts. The current flows through the regulating resistance R and 
through the audio choke L1 to the carbons; returning through the ammeter A and 
the second audio choke L2. The chokes and resistor should be cap- of passing 
the necessary current without becoming too hot. The carbons are a quarter of 
an inch thick, or so.

The part of the circuit  at the right -- including the condenser C and coil L, 
with the hot-wire ammeter HA- is switched  in later The capacity of C is.; 4 
mf. and its rating  should be at least 700-volt D.C. test; the coil L is 2 
inches in diameter and has 60 turns of No. 18 wire, suitably insulated. (The 
frequency of this tuned circuit falls within the audible range.)

The experiment begins by bringing the two carbons in contact and separating 
them a small fraction of an inch; thus starting the arc. The resistor, R, is 
set to give a suitable current flow; as much as 2 amperes may be l readily 
drawn from the light-lines.

If  w e now switch  in the resonant  ciruit C-L, the arc gives out a whistling 
 note. At the same time, we see, the hot- e ammeter HA gives a kick. Since 
this circuit  is blocked to direct current, by the blocking condenser, C, it 
is evident that the cause must he a flow of alternating em rent. Therefore we 
know that the circuit is oscillating, and the current in it is relatively 
great; in the writer's experiments it has amounted to 4 amperes, registered by 
the hot-wire ammeter.

Why the Arc "Sings"

The cause of the sound deserves explanation. When the resonant circuit is 
shocked into oscillation, alternating current ale set up, and flow over the 
leads into the carbons; they cannot pass out into the feed-lines, because the 
chokes L1 and L2 stop their flow. Consequently, the alternating current is 
superimposed on the direct current in the carbons, e causing alternate 
strengthening and weakening. This also causes  a fluctuation in the emission 
of ionized gas from  the arc; and the temperature of the air about it 
fluctuates also, setting up successive compressions and rarefactions, which we 
term "sound waves", and which our ears recognize.

The small Tesla coil used by the author, with its coronal discharge from the 
high-voltage end: the large primary is for connection to a high-voltage 
generator.

At the top, the fundamental circuit of the arc-light generator, which came 
before the vacuum tube. Below. Various experiments with the circuit.

The layout Of the equipment used in these high-voltage. high-frequency 
experiments The condenser and coil determine the tuning of the circuit in 
which the arc sets up oscillations At the right, experiments in the field of 
the coil, and method of keeping arc in atmosphere of hydrocarbons.

Our second experiment is to vary the capacity of the condenser C; if it is 
replaced by one of 2-MF., the note becomes higher, with an increase in the 
frequency of the oscillating circuit. We may, as In Fig. 2A, substitute a 
group of condensers, 50 that we can switch from one to another; the pitch will 
change.

5imilarly. as in Fig. 2B, we may substitute coils with d referent numbers of 
turns: the altered inductance also affecting the pitch o! the note emitted by 
the arc. More inductance (that is, more turns. on the same diameter with same 
spacing) the deeper the note,

If we substitute for the 4-mf. condenser with which we started, a radio tuning 
condenser, with a value of .0005-mf., for instance, there is no oscillation in 
the radio-frequency resonating circuit thus produced, Not only will there be 
no audible note, but the hot-wire ammeter will register no current. We will 
deal smith this phenomenon further on.

HIGH-FREQUENCY TEST

All alternating currents have the property of causing magnetic coupling 
between circuits. If we make two turns of heavier copper wire, say No. 12, two 
inches in diameter., and connect the ends across a small flashlight bulb, as 
its detail A, attach an insulating handle; and approach the resonating coil L. 
with the exploring coil held parallel to the windings of L, we shall find that 
the lamp lights up brilliantly.

We can also increase the inductance of the coil L by introducing into its 
center a piece of iron rod. as shown in detail B. This ``ill at once produce a 
decided change in the pitch of the tone emitted by the arc.

Alternating currents produce, by induction, local currents in metal masses of 
any shape or form. The presence of these "eddy currents" may be demonstrated 
by putting a few small nails or screws in a test tube. covering them with 
water, and introducing the tube into the field of the coil L (Detail C). Not 
only will this have its effect on the tone of the arc: but after a few minutes 
we shall find the glass warmed by the heat generated by the eddy currents ire 
the metal, anal transmitted by the water, (This is the principle of the 
high-frequency furnace.)

REACHING RADIO FREQUENCIES

As said before. we failed to generate ratio frequency current by the coil when 
we reduced the capacity of the condenser to a proper value. Even a, .0001-mf., 
the writer failed to obtain a reading on the hot-wire ammeter.

However, we can easily obtain a positive result here; we have only to let the 
carbons burn in an atmosphere of hydrocarbons. This is easily obtained by 
placing tinder the arc a Bunsen burner, with the air holes at its base closed.

If we now replace the big condensers by a variable condenser, of .0005-mf., or 
ogle of the older 001-mf. instruments, then our hot-wire ammeter at once 
registers. Also, our exploring coil will show that high-frequency current is 
being generated in the oscillating circuit. There is. however. no whistle from 
the are: eve are in the range of radio-frequency vibrations, above the audio 
range. We have, in fact, a miniature "arc transmitter".

The resonance of radio circuits. similarly tuned. is at the basis of radio 
transmission and reception. We can illustrate the principle. as in Fig. 3. by 
setting up a similar oscillating circuit with a coil. LE, wound like L, and 
with a similar variable condenser across its ends in series with a small neon 
lamp. When the circuits are tuners alike. the bulb lights up. It makes no 
difference whether eve tune the receiver to the transmitter, or the 
transmitter to the receive en

If we take the receiving circuit further away, we thus reduce the coupling. 
and increase the sharpness of response. If we come closer, the sharpness is 
reduced, but the current received is much greater.

We may place the test coil some distance away, at right angles to the 
oscillating coil L, and make a direct coupling with a piece of wire, as in 
Fig. 4: the lamp will light. The effect of tinning may be again demonstrated.

Of a condenser, to introduce capacity. The distributed capacity of a winding 
may serve, as in the well-known Tesla coils.

For these experiments, it is easy to make a Tesla coil. by winding on any 
suitable insulating tube, 14 inches long, anti 11/4 inches in diameter. a full 
layer of No. 32 D.S.C. wire. This is close wound, and the spool is covered 
with a suitable lacquer, or collodion. to hold the windings in place.

The beginning of the winding is grounded. through a binding-post attached to 
its base: and the other end iri connected to a brass ball, at the top of the 
tube-.15 illustrated in the photograph reproduced at the head of this article. 
The base holds the coil upright.

Such a coil has considerable self-capaciry. between its respective windings, 
one with another. which reaches a very respectable total. This capacity makes 
it possible for the whole coil to have a natural fundamental frequency. In 
response to excitation. this is capable of oscillating: as we shall proceed to 
demonstrate by coupling it to the arc generator.

The arc has been introduced into an atmosphere of gas (Detail D) as explained 
above The capacity C is now represented by a .001-mf. Variable condenser: whim 
the coil L has now been replaced by five turns of No. 12 heavy bare copper 
wire. 8 inches in diameter, and almost self-supporting.

The Tesla coil is placed in the center. as shown in our first illustration, 
with its "foot" grounded.

The current through the Tesla coil will he too small to light a filament-type 
incandescent lamp; but the alternating voltage at the terminal of the coil is 
very high, and we can utilize this fact to light a neon lamp-one of those. for 
instance, used to test a radio receiver. We apply the test prod of such a neon 
lamp to the ball at the top of our Tesla coil.

A, soon as the oscillating circuit of our arc-generator is tuned to the 
fundamental frequency or the Tesla coil, the lamp applied to the top of the 
latter will light brilliantly with the rushes of current back and forth, into 
and front this capacity (Fig. 5).

We may also. if we wish. test direct. Instead of inductive, coupling to the 
Tesla coil. as we did before with our external tuned (receiving) circuit. This 
is shown in fig. 6, where the coil L is common to both the Tesla coil's 
circuit and that of the arc-generator circuit. Grounding is effected now 
through a 1-mf. fixed condenser C. As before, the neon lamp will light up, to 
denote the oscillating condition in the Tesla coil.

The test lamp lights, proving that the Tesla coil is oscillating at high 
frequency,

 666	EVERYDAY SCIENCE AND MECHANICS for JUNE, 1932


 
Experiments with "Singing
and Tesla Coil

By W. MOELLER (Germany)

SINCE radio discovery y began the methods used to generate high fre- have often changed. There was first Hertz's   resonant circuit excited by sparks; this method, while applicable to telegraphy creates only damped waves unsuitable for modulation In 1900 while seeking a method of created  undamped  waves, Duddell\ discovered  the "singing arc" he demonstrated that an arc light is capable of exciting continuous oscillation in a parallel  resonant circuit. In 1906 Valdemar  Poulsen constructed the  first practical arc transmitter for transmissionwork; but it was soon superseded by the vacuum-tube transmitter.

However, students of  radio development will find experiments with the arc-light generator  quite interesting; and they can be performed with com- simple means, such as a small laboratory  affords The first is Duddell's classic experiment, showing  the generation of alternating current in any circuit containing capacity and inductance as soon as it is connected across an electric arc

An apparatus like that illustrated in the center, below and corresponding to the schematic circuit of Fig. 1, i:; set up. The direct-current supply should be about 200 volts. The current flows through the regulating resistance R and through the audio choke L1 to the carbons; returning through the ammeter A and the second audio choke L2. The chokes and resistor should be cap- of passing the necessary current without becoming too hot. The carbons are a quarter of an inch thick, or so.

The part of the circuit  at the right -- including the condenser C and coil L, with the hot-wire ammeter HA- is switched  in later The capacity of C is.; 4 mf. and its rating  should be at least 700-volt D.C. test; the coil L is 2 inches in diameter and has 60 turns of No. 18 wire, suitably insulated. (The frequency of this tuned circuit falls within the audible range.)

The experiment begins by bringing the two carbons in contact and separating them a small fraction of an inch; thus starting the arc. The resistor, R, is set to give a suitable current flow; as much as 2 amperes may be l readily drawn from the light-lines.

If  w e now switch  in the resonant  ciruit C-L, the arc gives out a whistling  note. At the same time, we see, the hot- e ammeter HA gives a kick. Since this circuit  is blocked to direct current, by the blocking condenser, C, it is evident that the cause must he a flow of alternating em rent. Therefore we know that the circuit is oscillating, and the current in it is relatively great; in the writer's experiments it has amounted to 4 amperes, registered by the hot-wire ammeter.

Why the Arc "Sings"

The cause of the sound deserves explanation. When the resonant circuit is shocked into oscillation, alternating current ale set up, and flow over the leads into the carbons; they cannot pass out into the feed-lines, because the chokes L1 and L2 stop their flow. Consequently, the alternating current is superimposed on the direct current in the carbons, e causing alternate strengthening and weakening. This also causes  a fluctuation in the emission of ionized gas from  the arc; and the temperature of the air about it fluctuates also, setting up successive compressions and rarefactions, which we term "sound waves", and which our ears recognize.

The small Tesla coil used by the author, with its coronal discharge from the high-voltage end: the large primary is for connection to a high-voltage generator.

At the top, the fundamental circuit of the arc-light generator, which came before the vacuum tube. Below. Various experiments with the circuit.

The layout Of the equipment used in these high-voltage. high-frequency experiments The condenser and coil determine the tuning of the circuit in which the arc sets up oscillations At the right, experiments in the field of the coil, and method of keeping arc in atmosphere of hydrocarbons.

Our second experiment is to vary the capacity of the condenser C; if it is replaced by one of 2-MF., the note becomes higher, with an increase in the frequency of the oscillating circuit. We may, as In Fig. 2A, substitute a group of condensers, 50 that we can switch from one to another; the pitch will change.

5imilarly. as in Fig. 2B, we may substitute coils with d referent numbers of turns: the altered inductance also affecting the pitch o! the note emitted by the arc. More inductance (that is, more turns. on the same diameter with same spacing) the deeper the note,

If we substitute for the 4-mf. condenser with which we started, a radio tuning condenser, with a value of .0005-mf., for instance, there is no oscillation in the radio-frequency resonating circuit thus produced, Not only will there be no audible note, but the hot-wire ammeter will register no current. We will deal smith this phenomenon further on.

HIGH-FREQUENCY TEST

All alternating currents have the property of causing magnetic coupling between circuits. If we make two turns of heavier copper wire, say No. 12, two inches in diameter., and connect the ends across a small flashlight bulb, as its detail A, attach an insulating handle; and approach the resonating coil L. with the exploring coil held parallel to the windings of L, we shall find that the lamp lights up brilliantly.

We can also increase the inductance of the coil L by introducing into its center a piece of iron rod. as shown in detail B. This ``ill at once produce a decided change in the pitch of the tone emitted by the arc.

Alternating currents produce, by induction, local currents in metal masses of any shape or form. The presence of these "eddy currents" may be demonstrated by putting a few small nails or screws in a test tube. covering them with water, and introducing the tube into the field of the coil L (Detail C). Not only will this have its effect on the tone of the arc: but after a few minutes we shall find the glass warmed by the heat generated by the eddy currents ire the metal, anal transmitted by the water, (This is the principle of the high-frequency furnace.)

REACHING RADIO FREQUENCIES

As said before. we failed to generate ratio frequency current by the coil when we reduced the capacity of the condenser to a proper value. Even a, .0001-mf., the writer failed to obtain a reading on the hot-wire ammeter.

However, we can easily obtain a positive result here; we have only to let the carbons burn in an atmosphere of hydrocarbons. This is easily obtained by placing tinder the arc a Bunsen burner, with the air holes at its base closed.

If we now replace the big condensers by a variable condenser, of .0005-mf., or ogle of the older 001-mf. instruments, then our hot-wire ammeter at once registers. Also, our exploring coil will show that high-frequency current is being generated in the oscillating circuit. There is. however. no whistle from the are: eve are in the range of radio-frequency vibrations, above the audio range. We have, in fact, a miniature "arc transmitter".

The resonance of radio circuits. similarly tuned. is at the basis of radio transmission and reception. We can illustrate the principle. as in Fig. 3. by setting up a similar oscillating circuit with a coil. LE, wound like L, and with a similar variable condenser across its ends in series with a small neon lamp. When the circuits are tuners alike. the bulb lights up. It makes no difference whether eve tune the receiver to the transmitter, or the transmitter to the receive en

If we take the receiving circuit further away, we thus reduce the coupling. and increase the sharpness of response. If we come closer, the sharpness is reduced, but the current received is much greater.

We may place the test coil some distance away, at right angles to the oscillating coil L, and make a direct coupling with a piece of wire, as in Fig. 4: the lamp will light. The effect of tinning may be again demonstrated.

Of a condenser, to introduce capacity. The distributed capacity of a winding may serve, as in the well-known Tesla coils.

For these experiments, it is easy to make a Tesla coil. by winding on any suitable insulating tube, 14 inches long, anti 11/4 inches in diameter. a full layer of No. 32 D.S.C. wire. This is close wound, and the spool is covered with a suitable lacquer, or collodion. to hold the windings in place.

The beginning of the winding is grounded. through a binding-post attached to its base: and the other end iri connected to a brass ball, at the top of the tube-.15 illustrated in the photograph reproduced at the head of this article. The base holds the coil upright.

Such a coil has considerable self-capaciry. between its respective windings, one with another. which reaches a very respectable total. This capacity makes it possible for the whole coil to have a natural fundamental frequency. In response to excitation. this is capable of oscillating: as we shall proceed to demonstrate by coupling it to the arc generator.

The arc has been introduced into an atmosphere of gas (Detail D) as explained above The capacity C is now represented by a .001-mf. Variable condenser: whim the coil L has now been replaced by five turns of No. 12 heavy bare copper wire. 8 inches in diameter, and almost self-supporting.

The Tesla coil is placed in the center. as shown in our first illustration, with its "foot" grounded.

The current through the Tesla coil will he too small to light a filament-type incandescent lamp; but the alternating voltage at the terminal of the coil is very high, and we can utilize this fact to light a neon lamp-one of those. for instance, used to test a radio receiver. We apply the test prod of such a neon lamp to the ball at the top of our Tesla coil.

A, soon as the oscillating circuit of our arc-generator is tuned to the fundamental frequency or the Tesla coil, the lamp applied to the top of the latter will light brilliantly with the rushes of current back and forth, into and front this capacity (Fig. 5).

We may also. if we wish. test direct. Instead of inductive, coupling to the Tesla coil. as we did before with our external tuned (receiving) circuit. This is shown in fig. 6, where the coil L is common to both the Tesla coil's circuit and that of the arc-generator circuit. Grounding is effected now through a 1-mf. fixed condenser C. As before, the neon lamp will light up, to denote the oscillating condition in the Tesla coil.

The test lamp lights, proving that the Tesla coil is oscillating at high frequency,

 666	EVERYDAY SCIENCE AND MECHANICS for JUNE, 1932