subjected to neutralizing or intensifying effects of the current in the
shunt through _c'_, and the magnetism of the cores will be partially
neutralized, or the points of greatest magnetism shifted, so that it
will be more or less remote from or approaching to the armature, and
hence the aggregate energizing actions of the field magnets on the
armature will be correspondingly varied.
In the form indicated in Fig. 253 the regulation is effected by shifting
the point of greatest magnetism, and in Figs. 254 and 255 the same
effect is produced by the action of the current in the shunt passing
through the neutralizing helix.
The relative positions of the respective brushes may be varied by moving
the auxiliary brush, or the brush _c_ may remain stationary and the core
P be connected to the main-brush holder A, so as to adjust the brushes
_a b_ in their relation to the brush _c_. If, however, an adjustment is
applied to all the brushes, as seen in Fig. 257, the solenoid should be
connected to both _a_ and _c_, so as to move them toward or away from
each other.
There are several known devices for giving motion in proportion to an
electric current. In Figs. 256 and 257 the moving cores are shown as
convenient devices for obtaining the required extent of motion with very
slight changes in the current passing through the helices. It is
understood that the adjustment of the main brushes causes variations in
the strength of the current independently of the relative position of
those brushes to the auxiliary brush. In all cases the adjustment should
be such that no current flows over the auxiliary brush when the dynamo
is running with its normal load.
In Figs. 256 and 257 A A indicate the main-brush holder, carrying the
main brushes, and C the auxiliary-brush holder, carrying the auxiliary
brush. These brush-holders are movable in arcs concentric with the
centre of the commutator-shaft. An iron piston, P, of the solenoid S,
Fig. 256, is attached to the auxiliary-brush holder C. The adjustment is
effected by means of a spring and screw or tightener.
In Fig. 257 instead of a solenoid, an iron tube inclosing a coil is
shown. The piston of the coil is attached to both brush-holders A A and
C. When the brushes are moved directly by electrical devices, as shown
in Figs. 256 and 257, these are so constructed that the force exerted
for adjusting is practically uniform through the whole length of motion.
[Illustration: FIG. 255.]
It is true that auxiliary brushes have been used in connection with the
helices of the field-wire; but in these instances the helices receive
the entire current through the auxiliary brush or brushes, and these
brushes could not be taken off without breaking the circuit through the
field. These brushes cause, moreover, heavy sparking at the commutator.
In the present case the auxiliary brush causes very little or no
sparking, and can be taken off without breaking the circuit through the
field-helices. The arrangement has, besides, the advantage of
facilitating the self-excitation of the machine in all cases where the
resistance of the field-wire is very great comparatively to the
resistance of the main circuit at the start--for instance, on arc-light
machines. In this case the auxiliary brush _c_ is placed near to, or
better still in contact with, the brush _b_, as shown in Fig. 258. In
this manner the part M' is completely cut out, and as the part M has a
considerably smaller resistance than the whole length of the field-wire
the machine excites itself, whereupon the auxiliary brush is shifted
automatically to its normal position.
[Illustration: FIG. 256.]
[Illustration: FIG. 257.]
In a further method devised by Mr. Tesla, one or more auxiliary brushes
are employed, by means of which a portion or the whole of the field
coils is shunted. According to the relative position upon the commutator
of the respective brushes more or less current is caused to pass through
the helices of the field, and the current developed by the machine can
be varied at will by varying the relative positions of the brushes.
[Illustration: FIG. 258.]
In Fig. 259, _a_ and _b_ are the positive and negative brushes of the
main circuit, and _c_ an auxiliary brush. The main circuit D extends
from the brushes _a_ and _b_, as usual, and contains the helices M of
the field wire and the electric lamps or other working devices. The
auxiliary brush _c_ is connected to the point _x_ of the main circuit by
means of the wire _c'_. H is a commutator of ordinary construction. It
will have been seen from what was said already that when the
electro-motive force between the brushes _a_ and _c_ is to the
electromotive force between the brushes _c_ and _b_ as the resistance of
the circuit _a_ M _c' c_ A is to the resistance of the circuit _b_ C B
_c c'_ D, the potentials of the points _x_ and _y_ will be equal, and no
current will pass over the auxiliary brush _c_; but if that brush
occupies a different position relatively to the main brushes the
electric condition is disturbed, and current will flow either from _y_
to _x_ or from _x_ to _y_, according to the relative position of the
brushes. In the first case the current through the field-helices will be
partly neutralized and the magnetism of the field magnets will be
diminished. In the second case the current will be increased and the
magnets gain strength. By combining with the brushes at _a b c_ any
automatic regulating mechanism, the current developed can be regulated
automatically in proportion to the demands of the working circuit.
In Figs. 264 and 265 some of the automatic means are represented that
maybe used for moving the brushes. The core P, Fig. 264, of the
solenoid-helix S is connected with the brush _a_ to move the same, and
in Fig. 265 the core P is shown as within the helix S, and connected
with brushes _a_ and _c_, so as to move the same toward or from each
other, according to the strength of the current in the helix, the helix
being within an iron tube, S', that becomes magnetized and increases the
action of the solenoid.
In practice it is sufficient to move only the auxiliary brush, as shown
in Fig. 264, as the regulation is very sensitive to the slightest
changes; but the relative position of the auxiliary brush to the main
brushes may be varied by moving the main brushes, or both main and
auxiliary brushes may be moved, as illustrated in Fig. 265. In the
latter two cases, it will be understood, the motion of the main brushes
relatively to the neutral line of the machine causes variations in the
strength of the current independently of their relative position to the
auxiliary brush. In all cases the adjustment may be such that when the
machine is running with the ordinary load, no current flows over the
auxiliary brush.
The field helices may be connected, as shown in Fig. 259, or a part of
the field helices may be in the outgoing and the other part in the
return circuit, and two auxiliary brushes may be employed as shown in
Figs. 261 and 262. Instead of shunting the whole of the field helices, a
portion only of such helices may be shunted, as shown in Figs. 260 and
262.
The arrangement shown in Fig. 262 is advantageous, as it diminishes the
sparking upon the commutator, the main circuit being closed through the
auxiliary brushes at the moment of the break of the circuit at the main
brushes.
[Illustration: FIG. 259.]
[Illustration: FIG. 260.]
[Illustration: FIG. 261.]
[Illustration: FIG. 262.]
[Illustration: FIG. 263.]
The field helices may be wound in the same direction, or a part may be
wound in opposite directions.
The connection between the helices and the auxiliary brush or brushes
may be made by a wire of small resistance, or a resistance may be
interposed (R, Fig. 263,) between the point _x_ and the auxiliary brush
or brushes to divide the sensitiveness when the brushes are adjusted.
[Illustration: FIG. 264.]
[Illustration: FIG. 265.]
The accompanying sketches also illustrate improvements made by Mr. Tesla
in the mechanical devices used to effect the shifting of the brushes, in
the use of an auxiliary brush. Fig. 266 is an elevation of the regulator
with the frame partly in section; and Fig. 267 is a section at the line
_x x_, Fig. 266. C is the commutator; B and B', the brush-holders, B
carrying the main brushes _a a'_, and B' the auxiliary or shunt brushes
_b b_. The axis of the brush-holder B is supported by two pivot-screws,
_p p_. The other brush-holder, B', has a sleeve, _d_, and is movable
around the axis of the brush-holder B. In this way both brush-holders
can turn very freely, the friction of the parts being reduced to a
minimum. Over the brush-holders is mounted the solenoid S, which rests
upon a forked column, _c_. This column also affords a support for the
pivots _p p_, and is fastened upon a solid bracket or projection, P,
which extends from the base of the machine, and is cast in one piece
with the same. The brush-holders B B' are connected by means of the
links _e e_ and the cross-piece F to the iron core I, which slides
freely in the tube T of the solenoid. The iron core I has a screw, _s_,
by means of which it can be raised and adjusted in its position
relatively to the solenoid, so that the pull exerted upon it by the
solenoid is practically uniform through the whole length of motion which
is required to effect the regulation. In order to effect the adjustment
with greater precision, the core I is provided with a small iron screw,
_s'_. The core being first brought very nearly in the required position
relatively to the solenoid by means of the screw _s_, the small screw
_s'_ is then adjusted until the magnetic attraction upon the core is the
same when the core is in any position. A convenient stop, _t_, serves to
limit the upward movement of the iron core.
To check somewhat the movement of the core I, a dash-pot, K, is used.
The piston L of the dash-pot is provided with a valve, V, which opens by
a downward pressure and allows an easy downward movement of the iron
core I, but closes and checks the movement of the core when it is pulled
up by the action of the solenoid.
To balance the opposing forces, the weight of the moving parts, and the
pull exerted by the solenoid upon the iron core, the weights W W may be
used. The adjustment is such that when the solenoid is traversed by the
normal current it is just strong enough to balance the downward pull of
the parts.
[Illustration: FIG. 266.]
[Illustration: FIG. 267.]
The electrical circuit-connections are substantially the same as
indicated in the previous diagrams, the solenoid being in series with
the circuit when the translating devices are in series, and in shunt
when the devices are in multiple arc. The operation of the device is as
follows: When upon a decrease of the resistance of the circuit or for
some other reason, the current is increased, the solenoid S gains in
strength and pulls up the iron core I, thus shifting the main brushes in
the direction of rotation and the auxiliary brushes in the opposite way.
This diminishes the strength of the current until the opposing forces
are balanced and the solenoid is traversed by the normal current; but if
from any cause the current in the circuit is diminished, then the weight
of the moving parts overcomes the pull of the solenoid, the iron core I
descends, thus shifting the brushes the opposite way and increasing the
current to the normal strength. The dash-pot connected to the iron core
I may be of ordinary construction; but it is better, especially in
machines for arc lights, to provide the piston of the dash-pot with a
valve, as indicated in the diagrams. This valve permits a comparatively
easy downward movement of the iron core, but checks its movement when it
is drawn up by the solenoid. Such an arrangement has the advantage that
a great number of lights may be put on without diminishing the
light-power of the lamps in the circuit, as the brushes assume at once
the proper position. When lights are cut out, the dash-pot acts to
retard the movement; but if the current is considerably increased the
solenoid gets abnormally strong and the brushes are shifted instantly.
The regulator being properly adjusted, lights or other devices may be
put on or out with scarcely any perceptible difference. It is obvious
that instead of the dash-pot any other retarding device may be used.
CHAPTER XXXIX.
IMPROVEMENT IN THE CONSTRUCTION OF DYNAMOS AND MOTORS.
This invention of Mr. Tesla is an improvement in the construction of
dynamo or magneto electric machines or motors, consisting in a novel
form of frame and field magnet which renders the machine more solid and
compact as a structure, which requires fewer parts, and which involves
less trouble and expense in its manufacture. It is applicable to
generators and motors generally, not only to those which have
independent circuits adapted for use in the Tesla alternating current
system, but to other continuous or alternating current machines of the
ordinary type generally used.
Fig. 268 shows the machine in side elevation. Fig. 269 is a vertical
sectional view of the field magnets and frame and an end view of the
armature; and Fig. 270 is a plan view of one of the parts of the frame
and the armature, a portion of the latter being cut away.
The field magnets and frame are cast in two parts. These parts are
identical in size and shape, and each consists of the solid plates or
ends A B, from which project inwardly the cores C D and the side bars or
bridge pieces, E F. The precise shape of these parts is largely a matter
of choice--that is to say, each casting, as shown, forms an
approximately rectangular frame; but it might obviously be more or less
oval, round, or square, without departure from the invention. It is also
desirable to reduce the width of the side bars, E F, at the center and
to so proportion the parts that when the frame is put together the
spaces between the pole pieces will be practically equal to the arcs
which the surfaces of the poles occupy.
The bearings G for the armature shaft are cast in the side bars E F. The
field coils are either wound on the pole pieces or on a form and then
slipped on over the ends of the pole pieces. The lower part or casting
is secured to the base after being finished off. The armature K on its
shaft is then mounted in the bearings of the lower casting and the
other part of the frame placed in position, dowel pins L or any other
means being used to secure the two parts in proper position.
[Illustration: FIG. 268.]
[Illustration: FIG. 269.]
[Illustration: FIG. 270.]
In order to secure an easier fit, the side bars E F, and end pieces, A
B, are so cast that slots M are formed when the two parts are put
together.
This machine possesses several advantages. For example, if we magnetize
the cores alternately, as indicated by the characters N S, it will be
seen that the magnetic circuit between the poles of each part of a
casting is completed through the solid iron side bars. The bearings for
the shaft are located at the neutral points of the field, so that the
armature core is not affected by the magnetic condition of the field.
The improvement is not restricted to the use of four pole pieces, as it
is evident that each pole piece could be divided or more than four
formed by the shape of the casting.
CHAPTER XL.
TESLA DIRECT CURRENT ARC LIGHTING SYSTEM.
At one time, soon after his arrival in America, Mr. Tesla was greatly
interested in the subject of arc lighting, which then occupied public
attention and readily enlisted the support of capital. He therefore
worked out a system which was confided to a company formed for its
exploitation, and then proceeded to devote his energies to the
perfection of the details of his more celebrated "rotary field" motor
system. The Tesla arc lighting apparatus appeared at a time when a great
many other lamps and machines were in the market, but it commanded
notice by its ingenuity. Its chief purpose was to lessen the
manufacturing cost and simplify the processes of operation.
We will take up the dynamo first. Fig. 271 is a longitudinal section,
and Fig. 272 a cross section of the machine. Fig. 273 is a top view, and
Fig. 274 a side view of the magnetic frame. Fig. 275 is an end view of
the commutator bars, and Fig. 276 is a section of the shaft and
commutator bars. Fig. 277 is a diagram illustrating the coils of the
armature and the connections to the commutator plates.
The cores _c c c c_ of the field-magnets are tapering in both
directions, as shown, for the purposes of concentrating the magnetism
upon the middle of the pole-pieces.
The connecting-frame F F of the field-magnets is in the form indicated
in the side view, Fig. 274, the lower part being provided with the
spreading curved cast legs _e e_, so that the machine will rest firmly
upon two base-bars, _r r_.
To the lower pole, S, of the field-magnet M is fastened, by means of
babbitt or other fusible diamagnetic material, the base B, which is
provided with bearings _b_ for the armature-shaft H. The base B has a
projection, P, which supports the brush-holders and the regulating
devices, which are of a special character devised by Mr. Tesla.
The armature is constructed with the view to reduce to a minimum the
loss of power due to Foucault currents and to the change of polarity,
and also to shorten as much as possible the length of the inactive wire
wound upon the armature core.
[Illustration: FIG. 271.]
It is well known that when the armature is revolved between the poles of
the field-magnets, currents are generated in the iron body of the
armature which develop heat, and consequently cause a waste of power.
Owing to the mutual action of the lines of force, the magnetic
properties of iron, and the speed of the different portions of the
armature core, these currents are generated principally on and near the
surface of the armature core, diminishing in strength gradually toward
the centre of the core. Their quantity is under some conditions
proportional to the length of the iron body in the direction in which
these currents are generated. By subdividing the iron core electrically
in this direction, the generation of these currents can be reduced to a
great extent. For instance, if the length of the armature-core is twelve
inches, and by a suitable construction it is subdivided electrically, so
that there are in the generating direction six inches of iron and six
inches of intervening air-spaces or insulating material, the waste
currents will be reduced to fifty per cent.
As shown in the diagrams, the armature is constructed of thin iron discs
D D D, of various diameters, fastened upon the armature-shaft in a
suitable manner and arranged according to their sizes, so that a series
of iron bodies, _i i i_, is formed, each of which diminishes in
thickness from the centre toward the periphery. At both ends of the
armature the inwardly curved discs _d d_, of cast iron, are fastened to
the armature shaft.
The armature core being constructed as shown, it will be easily seen
that on those portions of the armature that are the most remote from the
axis, and where the currents are principally developed, the length of
iron in the generating direction is only a small fraction of the total
length of the armature core, and besides this the iron body is
subdivided in the generating direction, and therefore the Foucault
currents are greatly reduced. Another cause of heating is the shifting
of the poles of the armature core. In consequence of the subdivision of
the iron in the armature and the increased surface for radiation, the
risk of heating is lessened.
The iron discs D D D are insulated or coated with some insulating-paint,
a very careful insulation being unnecessary, as an electrical contact
between several discs can only occur at places where the generated
currents are comparatively weak. An armature core constructed in the
manner described may be revolved between the poles of the field magnets
without showing the slightest increase of temperature.
[Illustration: FIG. 272.]
[Illustration: FIG. 273.]
The end discs, _d d_, which are of sufficient thickness and, for the
sake of cheapness, of cast-iron, are curved inwardly, as indicated in
the drawings. The extent of the curve is dependent on the amount of wire
to be wound upon the armatures. In this machine the wire is wound upon
the armature in two superimposed parts, and the curve of the end discs,
_d d_, is so calculated that the first part--that is, practically half
of the wire--just fills up the hollow space to the line _x x_; or, if
the wire is wound in any other manner, the curve is such that when the
whole of the wire is wound, the outside mass of wires, _w_, and the
inside mass of wires, _w'_, are equal at each side of the plane _x x_.
In this case the passive or electrically-inactive wires are of the
smallest length practicable. The arrangement has further the advantage
that the total lengths of the crossing wires at the two sides of the
plane _x x_ are practically equal.
[Illustration: FIG. 274.]
To equalize further the armature coils at both sides of the plates that
are in contact with the brushes, the winding and connecting up is
effected in the following manner: The whole wire is wound upon the
armature-core in two superimposed parts, which are thoroughly insulated
from each other. Each of these two parts is composed of three separated
groups of coils. The first group of coils of the first part of wire
being wound and connected to the commutator-bars in the usual manner,
this group is insulated and the second group wound; but the coils of
this second group, instead of being connected to the next following
commutator bars, are connected to the directly opposite bars of the
commutator. The second group is then insulated and the third group
wound, the coils of this group being connected to those bars to which
they would be connected in the usual way. The wires are then thoroughly
insulated and the second part of wire is wound and connected in the same
manner.
Suppose, for instance, that there are twenty-four coils--that is, twelve
in each part--and consequently twenty-four commutator plates. There will
be in each part three groups, each containing four coils, and the coils
will be connected as follows:
_Groups._ _Commutator Bars._
{ First 1--5
First part of wire { Second 17--21
{ Third 9--13
{ First 13--17
Second part of wire { Second 5--9
{ Third 21--1
In constructing the armature core and winding and connecting the coils
in the manner indicated, the passive or electrically inactive wire is
reduced to a minimum, and the coils at each side of the plates that are
in contact with the brushes are practically equal. In this way the
electrical efficiency of the machine is increased.
[Illustration: FIG. 275.]
[Illustration: FIG. 276.]
The commutator plates _t_ are shown as outside the bearing _b_ of the
armature shaft. The shaft H is tubular and split at the end portion, and
the wires are carried through the same in the usual manner and connected
to the respective commutator plates. The commutator plates are upon a
cylinder, _u_, and insulated, and this cylinder is properly placed and
then secured by expanding the split end of the shaft by a tapering screw
plug, _v_.
[Illustration: FIG. 277.]
The arc lamps invented by Mr. Tesla for use on the circuits from the
above described dynamo are those in which the separation and feed of the
carbon electrodes or their equivalents is accomplished by means of
electro-magnets or solenoids in connection with suitable clutch
mechanism, and were designed for the purpose of remedying certain
faults common to arc lamps.
He proposed to prevent the frequent vibrations of the movable carbon
"point" and flickering of the light arising therefrom; to prevent the
falling into contact of the carbons; to dispense with the dash pot,
clock work, or gearing and similar devices; to render the lamp extremely
sensitive, and to feed the carbon almost imperceptibly, and thereby
obtain a very steady and uniform light.
In that class of lamps where the regulation of the arc is effected by
forces acting in opposition on a free, movable rod or lever directly
connected with the electrode, all or some of the forces being dependent
on the strength of the current, any change in the electrical condition
of the circuit causes a vibration and a corresponding flicker in the
light. This difficulty is most apparent when there are only a few lamps
in circuit. To lessen this difficulty lamps have been constructed in
which the lever or armature, after the establishing of the arc, is kept
in a fixed position and cannot vibrate during the feed operation, the
feed mechanism acting independently; but in these lamps, when a clamp is
employed, it frequently occurs that the carbons come into contact and
the light is momentarily extinguished, and frequently parts of the
circuit are injured. In both these classes of lamps it has been
customary to use dash pot, clock work, or equivalent retarding devices;
but these are often unreliable and objectionable, and increase the cost
of construction.
Mr. Tesla combines two electro-magnets--one of low resistance in the
main or lamp circuit, and the other of comparatively high resistance in
a shunt around the arc--a movable armature lever, and a special feed
mechanism, the parts being arranged so that in the normal working
position of the armature lever the same is kept almost rigidly in one
position, and is not affected even by considerable changes in the
electric circuit; but if the carbons fall into contact the armature will
be actuated by the magnets so as to move the lever and start the arc,
and hold the carbons until the arc lengthens and the armature lever
returns to the normal position. After this the carbon rod holder is
released by the action of the feed mechanism, so as to feed the carbon
and restore the arc to its normal length.
Fig. 278 is an elevation of the mechanism made use of in this arc lamp.
Fig. 279 is a plan view. Fig. 280 is an elevation of the balancing lever
and spring; Fig. 281 is a detached plan view of the pole pieces and
armatures upon the friction clamp, and Fig. 282 is a section of the
clamping tube.
M is a helix of coarse wire in a circuit from the lower carbon holder to
the negative binding screw -. N is a helix of fine wire in a shunt
between the positive binding screw + and the negative binding screw -.
The upper carbon holder S is a parallel rod sliding through the plates
S' S^{2} of the frame of the lamp, and hence the electric current passes
from the positive binding post + through the plate S^{2}, carbon holder
S, and upper carbon to the lower carbon, and thence by the holder and a
metallic connection to the helix M.
[Illustration: FIG. 278.]
[Illustration: FIG. 279.]
[Illustration: FIG. 280.]
[Illustration: FIG. 281.]
[Illustration: FIG. 282.]
The carbon holders are of the usual character, and to insure electric
connections the springs _l_ are made use of to grasp the upper carbon
holding rod S, but to allow the rod to slide freely through the same.
These springs _l_ may be adjusted in their pressure by the screw _m_,
and the spring _l_ maybe sustained upon any suitable support. They are
shown as connected with the upper end of the core of the magnet N.
Around the carbon-holding rod S, between the plates S' S^{2}, there is a
tube, R, which forms a clamp. This tube is counter-bored, as seen in the
section Fig. 282, so that it bears upon the rod S at its upper end and
near the middle, and at the lower end of this tubular clamp R there are
armature segments _r_ of soft iron. A frame or arm, _n_, extending,
preferably, from the core N^{2}, supports the lever A by a fulcrum-pin,
_o_. This lever A has a hole, through which the upper end of the tubular
clamp R passes freely, and from the lever A is a link, _q_, to the lever
_t_, which lever is pivoted at _y_ to a ring upon one of the columns
S^{3}. This lever _t_ has an opening or bow surrounding the tubular
clamp R, and there are pins or pivotal connections _w_ between the lever
_t_ and this clamp R, and a spring, _r^{2}_, serves to support or
suspend the weight of the parts and balance them, or nearly so. This
spring is adjustable.
At one end of the lever A is a soft-iron armature block, _a_, over the
core M' of the helix M, and there is a limiting screw, _c_, passing
through this armature block _a_, and at the other end of the lever A is
a soft iron armature block, _b_, with the end tapering or wedge shaped,
and the same comes close to and in line with the lateral projection _e_
on the core N^{2}. The lower ends of the cores M' N^{2} are made with
laterally projecting pole-pieces M^{3} N^{3}, respectively, and these
pole-pieces are concave at their outer ends, and are at opposite sides
of the armature segments _r_ at the lower end of the tubular clamp R.
The operation of these devices is as follows: In the condition of
inaction, the upper carbon rests upon the lower one, and when the
electric current is turned on it passes freely, by the frame and spring
_l_, through the rods and carbons to the coarse wire and helix M, and to
the negative binding post V and the core M' thereby is energized. The
pole piece M^{3} attracts the armature _r_, and by the lateral pressure
causes the clamp R to grasp the rod S', and the lever A is
simultaneously moved from the position shown by dotted lines, Fig. 278,
to the normal position shown in full lines, and in so doing the link _q_
and lever _t_ are raised, lifting the clamp R and S, separating the
carbons and forming the arc. The magnetism of the pole piece _e_ tends
to hold the lever A level, or nearly so, the core N^{2} being energized
by the current in the shunt which contains the helix N. In this position
the lever A is not moved by any ordinary variation in the current,
because the armature _b_ is strongly attracted by the magnetism of _e_,
and these parts are close to each other, and the magnetism of _e_ acts
at right angles to the magnetism of the core M'. If, now, the arc
becomes too long, the current through the helix M is lessened, and the
magnetism of the core N^{3} is increased by the greater current passing
through the shunt, and this core N^{3}, attracting the segmental
armature _r_, lessens the hold of the clamp R upon the rod S, allowing
the latter to slide and lessen the length of the arc, which instantly
restores the magnetic equilibrium and causes the clamp R to hold the rod
S. If it happens that the carbons fall into contact, then the magnetism
of N^{2} is lessened so much that the attraction of the magnet M will be
sufficient to move the armature _a_ and lever A so that the armature _b_
passes above the normal position, so as to separate the carbons
instantly; but when the carbons burn away, a greater amount of current
will pass through the shunt until the attraction of the core N^{2} will
overcome the attraction of the core M' and bring the armature lever A
again into the normal horizontal position, and this occurs before the
feed can take place. The segmental armature pieces _r_ are shown as
nearly semicircular. They are square or of any other desired shape, the
ends of the pole pieces M^{3}, N^{3} being made to correspond in shape.
In a modification of this lamp, Mr. Tesla provided means for
automatically withdrawing a lamp from the circuit, or cutting it out
when, from a failure of the feed, the arc reached an abnormal length;
and also means for automatically reinserting such lamp in the circuit
when the rod drops and the carbons come into contact.
Fig. 283 is an elevation of the lamp with the case in section. Fig. 284
is a sectional plan at the line _x x_. Fig. 285 is an elevation, partly
in section, of the lamp at right angles to Fig. 283. Fig. 286 is a
sectional plan at the line _y y_ of Fig. 283. Fig. 287 is a section of
the clamp in about full size. Fig. 288 is a detached section
illustrating the connection of the spring to the lever that carries the
pivots of the clamp, and Fig. 289 is a diagram showing the
circuit-connections of the lamp.
In Fig. 283, M represents the main and N the shunt magnet, both securely
fastened to the base A, which with its side columns, S S, are cast in
one piece of brass or other diamagnetic material. To the magnets are
soldered or otherwise fastened the brass washers or discs _a a a a_.
Similar washers, _b b_, of fibre or other insulating material, serve to
insulate the wires from the brass washers.
The magnets M and N are made very flat, so that their width exceeds
three times their thickness, or even more. In this way a comparatively
small number of convolutions is sufficient to produce the required
magnetism, while a greater surface is offered for cooling off the wires.
[Illustration: FIG. 286.]
[Illustration: FIG. 283.]
[Illustration: FIG. 285.]
[Illustration: FIG. 284.]
[Illustration: FIG. 287.]
[Illustration: FIG. 288.]
The upper pole pieces, _m n_, of the magnets are curved, as indicated in
the drawings, Fig. 283. The lower pole pieces _m' n'_, are brought near
together, tapering toward the armature _g_, as shown in Figs. 284 and
286. The object of this taper is to concentrate the greatest amount of
the developed magnetism upon the armature, and also to allow the pull to
be exerted always upon the middle of the armature _g_. This armature _g_
is a piece of iron in the shape of a hollow cylinder, having on each
side a segment cut away, the width of which is equal to the width of the
pole pieces _m' n'_.
The armature is soldered or otherwise fastened to the clamp _r_, which
is formed of a brass tube, provided with gripping-jaws _e e_, Fig. 287.
These jaws are arcs of a circle of the diameter of the rod R, and are
made of hardened German silver. The guides _f f_, through which the
carbon-holding rod R slides, are made of the same material. This has the
advantage of reducing greatly the wear and corrosion of the parts coming
in frictional contact with the rod, which frequently causes trouble. The
jaws _e e_ are fastened to the inside of the tube _r_, so that one is a
little lower than the other. The object of this is to provide a greater
opening for the passage of the rod when the same is released by the
clamp. The clamp _r_ is supported on bearings _w w_, Figs. 283, 285 and
287, which are just in the middle between the jaws _e e_. The bearings
_w w_ are carried by a lever, _t_, one end of which rests upon an
adjustable support, _q_, of the side columns, S, the other end being
connected by means of the link _e'_ to the armature-lever L. The
armature-lever L is a flat piece of iron in N shape, having its ends
curved so as to correspond to the form of the upper pole-pieces of the
magnets M and N. It is hung upon the pivots _v v_, Fig. 284, which are
in the jaw _x_ of the top plate B. This plate B, with the jaw, is cast
in one piece and screwed to the side columns, S S, that extend up from
the base A. To partly balance the overweight of the moving parts, a
spring, _s'_, Figs. 284 and 288, is fastened to the top plate, B, and
hooked to the lever _t_. The hook _o_ is toward one side of the lever or
bent a little sidewise, as seen in Fig. 288. By this means a slight
tendency is given to swing the armature toward the pole-piece _m'_ of
the main magnet.
The binding-posts K K' are screwed to the base A. A manual switch, for
short-circuiting the lamp when the carbons are renewed, is also fastened
to the base. This switch is of ordinary character, and is not shown in
the drawings.
The rod R is electrically connected to the lamp-frame by means of a
flexible conductor or otherwise. The lamp-case receives a removable
cover, _s^{2}_, to inclose the parts.
The electrical connections are as indicated diagrammatically in Fig.
289. The wire in the main magnet consists of two parts, _x'_ and _p'_.
These two parts may be in two separated coils or in one single helix,
as shown in the drawings. The part _x'_ being normally in circuit, is,
with the fine wire upon the shunt-magnet, wound and traversed by the
current in the same direction, so as to tend to produce similar poles, N
N or S S, on the corresponding pole-pieces of the magnets M and N. The
part _p'_ is only in circuit when the lamp is cut out, and then the
current being in the opposite direction produces in the main magnet,
magnetism of the opposite polarity.
The operation is as follows: At the start the carbons are to be in
contact, and the current passes from the positive binding-post K to the
lamp-frame, carbon-holder, upper and lower carbon, insulated return-wire
in one of the side rods, and from there through the part _x'_ of the
wire on the main magnet to the negative binding-post. Upon the passage
of the current the main magnet is energized and attracts the
clamping-armature _g_, swinging the clamp and gripping the rod by means
of the gripping jaws _e e_. At the same time the armature lever L is
pulled down and the carbons are separated. In pulling down the armature
lever L the main magnet is assisted by the shunt-magnet N, the latter
being magnetized by magnetic induction from the magnet M.
[Illustration: FIG. 289.]
It will be seen that the armatures L and _g_ are practically the keepers
for the magnets M and N, and owing to this fact both magnets with either
one of the armatures L and _g_ may be considered as one horseshoe
magnet, which we might term a "compound magnet." The whole of the
soft-iron parts M, _m'_, _g_, _n'_, N and L form a compound magnet.
The carbons being separated, the fine wire receives a portion of the
current. Now, the magnetic induction from the magnet M is such as to
produce opposite poles on the corresponding ends of the magnet N; but
the current traversing the helices tends to produce similar poles on the
corresponding ends of both magnets, and therefore as soon as the fine
wire is traversed by sufficient current the magnetism of the whole
compound magnet is diminished.
With regard to the armature _g_ and the operation of the lamp, the pole
_m'_ may be considered as the "clamping" and the pole _n'_ as the
"releasing" pole.
As the carbons burn away, the fine wire receives more current and the
magnetism diminishes in proportion. This causes the armature lever L to
swing and the armature _g_ to descend gradually under the weight of the
moving parts until the end _p_, Fig. 283, strikes a stop on the top
plate, B. The adjustment is such that when this takes place the rod R is
yet gripped securely by the jaws _e e_. The further downward movement of
the armature lever being prevented, the arc becomes longer as the
carbons are consumed, and the compound magnet is weakened more and more
until the clamping armature _g_ releases the hold of the gripping-jaws
_e e_ upon the rod R, and the rod is allowed to drop a little, thus
shortening the arc. The fine wire now receiving less current, the
magnetism increases, and the rod is clamped again and slightly raised,
if necessary. This clamping and releasing of the rod continues until the
carbons are consumed. In practice the feed is so sensitive that for the
greatest part of the time the movement of the rod cannot be detected
without some actual measurement. During the normal operation of the lamp
the armature lever L remains practically stationary, in the position
shown in Fig. 283.
Should it happen that, owing to an imperfection in it, the rod and the
carbons drop too far, so as to make the arc too short, or even bring the
carbons in contact, a very small amount of current passes through the
fine wire, and the compound magnet becomes sufficiently strong to act as
at the start in pulling the armature lever L down and separating the
carbons to a greater distance.
It occurs often in practical work that the rod sticks in the guides. In
this case the are reaches a great length, until it finally breaks. Then
the light goes out, and frequently the fine wire is injured. To prevent
such an accident Mr. Tesla provides this lamp with an automatic cut-out
which operates as follows: When, upon a failure of the feed, the arc
reaches a certain predetermined length, such an amount of current is
diverted through the fine wire that the polarity of the compound magnet
is reversed. The clamping armature _g_ is now moved against the shunt
magnet N until it strikes the releasing pole _n'_. As soon as the
contact is established, the current passes from the positive binding
post over the clamp _r_, armature _g_, insulated shunt magnet, and the
helix _p'_ upon the main magnet M to the negative binding post. In this
case the current passes in the opposite direction and changes the
polarity of the magnet M, at the same time maintaining by magnetic
induction in the core of the shunt magnet the required magnetism without
reversal of polarity, and the armature _g_ remains against the shunt
magnet pole _n'_. The lamp is thus cut out as long as the carbons are
separated. The cut out may be used in this form without any further
improvement; but Mr. Tesla arranges it so that if the rod drops and the
carbons come in contact the arc is started again. For this purpose he
proportions the resistance of part _p'_ and the number of the
convolutions of the wire upon the main magnet so that when the carbons
come in contact a sufficient amount of current is diverted through the
carbons and the part _x'_ to destroy or neutralize the magnetism of the
compound magnet. Then the armature _g_, having a slight tendency to
approach to the clamping pole _m'_, comes out of contact with the
releasing pole _n'_. As soon as this happens, the current through the
part _p'_ is interrupted, and the whole current passes through the part
_x_. The magnet M is now strongly magnetized, the armature _g_ is
attracted, and the rod clamped. At the same time the armature lever L is
pulled down out of its normal position and the arc started. In this way
the lamp cuts itself out automatically when the arc gets too long, and
reinserts itself automatically in the circuit if the carbons drop
together.
CHAPTER XLI.
IMPROVEMENT IN "UNIPOLAR" GENERATORS.
Another interesting class of apparatus to which Mr. Tesla has directed
his attention, is that of "unipolar" generators, in which a disc or a
cylindrical conductor is mounted between magnetic poles adapted to
produce an approximately uniform field. In the disc armature machines
the currents induced in the rotating conductor flow from the centre to
the periphery, or conversely, according to the direction of rotation or
the lines of force as determined by the signs of the magnetic poles, and
these currents are taken off usually by connections or brushes applied
to the disc at points on its periphery and near its centre. In the case
of the cylindrical armature machine, the currents developed in the
cylinder are taken off by brushes applied to the sides of the cylinder
at its ends.
In order to develop economically an electromotive force available for
practicable purposes, it is necessary either to rotate the conductor at
a very high rate of speed or to use a disc of large diameter or a
cylinder of great length; but in either case it becomes difficult to
secure and maintain a good electrical connection between the collecting
brushes and the conductor, owing to the high peripheral speed.
It has been proposed to couple two or more discs together in series,
with the object of obtaining a higher electro-motive force; but with the
connections heretofore used and using other conditions of speed and
dimension of disc necessary to securing good practicable results, this
difficulty is still felt to be a serious obstacle to the use of this
kind of generator. These objections Mr. Tesla has sought to avoid by
constructing a machine with two fields, each having a rotary conductor
mounted between its poles. The same principle is involved in the case of
both forms of machine above described, but the description now given is
confined to the disc type, which Mr. Tesla is inclined to favor for that
machine. The discs are formed with flanges, after the manner of
pulleys, and are connected together by flexible conducting bands or
belts.
The machine is built in such manner that the direction of magnetism or
order of the poles in one field of force is opposite to that in the
other, so that rotation of the discs in the same direction develops a
current in one from centre to circumference and in the other from
circumference to centre. Contacts applied therefore to the shafts upon
which the discs are mounted form the terminals of a circuit the
electro-motive force in which is the sum of the electro-motive forces of
the two discs.
It will be obvious that if the direction of magnetism in both fields be
the same, the same result as above will be obtained by driving the discs
in opposite directions and crossing the connecting belts. In this way
the difficulty of securing and maintaining good contact with the
peripheries of the discs is avoided and a cheap and durable machine made
which is useful for many purposes--such as for an exciter for
alternating current generators, for a motor, and for any other purpose
for which dynamo machines are used.
[Illustration: FIG. 290.]
[Illustration: FIG. 291.]
Fig. 290 is a side view, partly in section, of this machine. Fig. 291 is
a vertical section of the same at right angles to the shafts.
In order to form a frame with two fields of force, a support, A, is cast
with two pole pieces B B' integral with it. To this are joined by bolts
E a casting D, with two similar and corresponding pole pieces C C'. The
pole pieces B B' are wound and connected to produce a field of force of
given polarity, and the pole pieces C C' are wound so as to produce a
field of opposite polarity. The driving shafts F G pass through the
poles and are journaled in insulating bearings in the casting A D, as
shown.
H K are the discs or generating conductors. They are composed of copper,
brass, or iron and are keyed or secured to their respective shafts. They
are provided with broad peripheral flanges J. It is of course obvious
that the discs may be insulated from their shafts, if so desired. A
flexible metallic belt L is passed over the flanges of the two discs,
and, if desired, may be used to drive one of the discs. It is better,
however, to use this belt merely as a conductor, and for this purpose
sheet steel, copper, or other suitable metal is used. Each shaft is
provided with a driving pulley M, by which power is imparted from a
driving shaft.
N N are the terminals. For the sake of clearness they are shown as
provided with springs P, that bear upon the ends of the shafts. This
machine, if self-exciting, would have copper bands around its poles; or
conductors of any kind--such as wires shown in the drawings--may be
used.
* * * * *
It is thought appropriate by the compiler to append here some notes on
unipolar dynamos, written by Mr. Tesla, on a recent occasion.
NOTES ON A UNIPOLAR DYNAMO.[15]
[15] Article by Mr. Tesla, contributed to _The Electrical Engineer_,
N. Y., Sept. 2, 1891.
It is characteristic of fundamental discoveries, of great achievements
of intellect, that they retain an undiminished power upon the
imagination of the thinker. The memorable experiment of Faraday with a
disc rotating between the two poles of a magnet, which has borne such
magnificent fruit, has long passed into every-day experience; yet there
are certain features about this embryo of the present dynamos and motors
which even to-day appear to us striking, and are worthy of the most
careful study.
Consider, for instance, the case of a disc of iron or other metal
revolving between the two opposite poles of a magnet, and the polar
surfaces completely covering both sides of the disc, and assume the
current to be taken off or conveyed to the same by contacts uniformly
from all points of the periphery of the disc. Take first the case of a
motor. In all ordinary motors the operation is dependent upon some
shifting or change of the resultant of the magnetic attraction exerted
upon the armature, this process being effected either by some mechanical
contrivance on the motor or by the action of currents of the proper
character. We may explain the operation of such a motor just as we can
that of a water-wheel. But in the above example of the disc surrounded
completely by the polar surfaces, there is no shifting of the magnetic
action, no change whatever, as far as we know, and yet rotation ensues.
Here, then, ordinary considerations do not apply; we cannot even give a
superficial explanation, as in ordinary motors, and the operation will
be clear to us only when we shall have recognized the very nature of the
forces concerned, and fathomed the mystery of the invisible connecting
mechanism.
Considered as a dynamo machine, the disc is an equally interesting
object of study. In addition to its peculiarity of giving currents of
one direction without the employment of commutating devices, such a
machine differs from ordinary dynamos in that there is no reaction
between armature and field. The armature current tends to set up a
magnetization at right angles to that of the field current, but since
the current is taken off uniformly from all points of the periphery, and
since, to be exact, the external circuit may also be arranged perfectly
symmetrical to the field magnet, no reaction can occur. This, however,
is true only as long as the magnets are weakly energized, for when the
magnets are more or less saturated, both magnetizations at right angles
seemingly interfere with each other.
For the above reason alone it would appear that the output of such a
machine should, for the same weight, be much greater than that of any
other machine in which the armature current tends to demagnetize the
field. The extraordinary output of the Forbes unipolar dynamo and the
experience of the writer confirm this view.
Again, the facility with which such a machine may be made to excite
itself is striking, but this may be due--besides to the absence of
armature reaction--to the perfect smoothness of the current and
non-existence of self-induction.
If the poles do not cover the disc completely on both sides, then, of
course, unless the disc be properly subdivided, the machine will be very
inefficient. Again, in this case there are points worthy of notice. If
the disc be rotated and the field current interrupted, the current
through the armature will continue to flow and the field magnets will
lose their strength comparatively slowly. The reason for this will at
once appear when we consider the direction of the currents set up in the
disc.
[Illustration: FIG. 292.]
Referring to the diagram Fig. 292, _d_ represents the disc with the
sliding contacts B B' on the shaft and periphery. N and S represent the
two poles of a magnet. If the pole N be above, as indicated in the
diagram, the disc being supposed to be in the plane of the paper, and
rotating in the direction of the arrow D, the current set up in the disc
will flow from the centre to the periphery, as indicated by the arrow A.
Since the magnetic action is more or less confined to the space between
the poles N S, the other portions of the disc may be considered
inactive. The current set up will therefore not wholly pass through the
external circuit F, but will close through the disc itself, and
generally, if the disposition be in any way similar to the one
illustrated, by far the greater portion of the current generated will
not appear externally, as the circuit F is practically short-circuited
by the inactive portions of the disc. The direction of the resulting
currents in the latter may be assumed to be as indicated by the dotted
lines and arrows _m_ and _n_; and the direction of the energizing field
current being indicated by the arrows _a b c d_, an inspection of the
figure shows that one of the two branches of the eddy current, that is,
A B' _m_ B, will tend to demagnetize the field, while the other branch,
that is, A B' _n_ B, will have the opposite effect. Therefore, the
branch A B' _m_ B, that is, the one which is _approaching_ the field,
will repel the lines of the same, while branch A B' _n_ B, that is, the
one _leaving_ the field, will gather the lines of force upon itself.
In consequence of this there will be a constant tendency to reduce the
current flow in the path A B' _m_ B, while on the other hand no such
opposition will exist in path A B' _n_ B, and the effect of the latter
branch or path will be more or less preponderating over that of the
former. The joint effect of both the assumed branch currents might be
represented by that of one single current of the same direction as that
energizing the field. In other words, the eddy currents circulating in
the disc will energize the field magnet. This is a result quite contrary
to what we might be led to suppose at first, for we would naturally
expect that the resulting effect of the armature currents would be such
as to oppose the field current, as generally occurs when a primary and
secondary conductor are placed in inductive relations to each other. But
it must be remembered that this results from the peculiar disposition in
this case, namely, two paths being afforded to the current, and the
latter selecting that path which offers the least opposition to its
flow. From this we see that the eddy currents flowing in the disc partly
energize the field, and for this reason when the field current is
interrupted the currents in the disc will continue to flow, and the
field magnet will lose its strength with comparative slowness and may
even retain a certain strength as long as the rotation of the disc is
continued.
The result will, of course, largely depend on the resistance and
geometrical dimensions of the path of the resulting eddy current and on
the speed of rotation; these elements, namely, determine the retardation
of this current and its position relative to the field. For a certain
speed there would be a maximum energizing action; then at higher speeds,
it would gradually fall off to zero and finally reverse, that is, the
resultant eddy current effect would be to weaken the field. The reaction
would be best demonstrated experimentally by arranging the fields N S,
N' S', freely movable on an axis concentric with the shaft of the disc.
If the latter were rotated as before in the direction of the arrow D,
the field would be dragged in the same direction with a torque, which,
up to a certain point, would go on increasing with the speed of
rotation, then fall off, and, passing through zero, finally become
negative; that is, the field would begin to rotate in opposite direction
to the disc. In experiments with alternate current motors in which the
field was shifted by currents of differing phase, this interesting
result was observed. For very low speeds of rotation of the field the
motor would show a torque of 900 lbs. or more, measured on a pulley 12
inches in diameter. When the speed of rotation of the poles was
increased, the torque would diminish, would finally go down to zero,
become negative, and then the armature would begin to rotate in opposite
direction to the field.
To return to the principal subject; assume the conditions to be such
that the eddy currents generated by the rotation of the disc strengthen
the field, and suppose the latter gradually removed while the disc is
kept rotating at an increased rate. The current, once started, may then
be sufficient to maintain itself and even increase in strength, and then
we have the case of Sir William Thomson's "current accumulator." But
from the above considerations it would seem that for the success of the
experiment the employment of a disc _not subdivided_[16] would be
essential, for if there should be a radial subdivision, the eddy
currents could not form and the self-exciting action would cease. If
such a radially subdivided disc were used it would be necessary to
connect the spokes by a conducting rim or in any proper manner so as to
form a symmetrical system of closed circuits.
[16] Mr. Tesla here refers to an interesting article which appeared
in July, 1865, in the _Phil. Magazine_, by Sir W. Thomson, in
which Sir William, speaking of his "uniform electric current
accumulator," assumes that for self-excitation it is desirable
to subdivide the disc into an infinite number of infinitely thin
spokes, in order to prevent diffusion of the current. Mr. Tesla
shows that diffusion is absolutely necessary for the excitation
and that when the disc is subdivided no excitation can occur.
The action of the eddy currents may be utilized to excite a machine of
any construction. For instance, in Figs. 293 and 294 an arrangement is
shown by which a machine with a disc armature might be excited. Here a
number of magnets, N S, N S, are placed radially on each side of a metal
disc D carrying on its rim a set of insulated coils, C C. The magnets
form two separate fields, an internal and an external one, the solid
disc rotating in the field nearest the axis, and the coils in the field
further from it. Assume the magnets slightly energized at the start;
they could be strengthened by the action of the eddy currents in the
solid disc so as to afford a stronger field for the peripheral coils.
Although there is no doubt that under proper conditions a machine might
be excited in this or a similar manner, there being sufficient
experimental evidence to warrant such an assertion, such a mode of
excitation would be wasteful.
But a unipolar dynamo or motor, such as shown in Fig. 292, may be
excited in an efficient manner by simply properly subdividing the disc
or cylinder in which the currents are set up, and it is practicable to
do away with the field coils which are usually employed. Such a plan is
illustrated in Fig. 295. The disc or cylinder D is supposed to be
arranged to rotate between the two poles N and S of a magnet, which
completely cover it on both sides, the contours of the disc and poles
being represented by the circles _d_ and _d^{1}_ respectively, the upper
pole being omitted for the sake of clearness. The cores of the magnet
are supposed to be hollow, the shaft C of the disc passing through them.
If the unmarked pole be below, and the disc be rotated screw fashion,
the current will be, as before, from the centre to the periphery, and
may be taken off by suitable sliding contacts, B B', on the shaft and
periphery respectively. In this arrangement the current flowing through
the disc and external circuit will have no appreciable effect on the
field magnet.
[Illustration: FIG. 293.]
[Illustration: FIG. 294.]
But let us now suppose the disc to be subdivided spirally, as indicated
by the full or dotted lines, Fig. 295. The difference of potential
between a point on the shaft and a point on the periphery will remain
unchanged, in sign as well as in amount. The only difference will be
that the resistance of the disc will be augmented and that there will be
a greater fall of potential from a point on the shaft to a point on the
periphery when the same current is traversing the external circuit. But
since the current is forced to follow the lines of subdivision, we see
that it will tend either to energize or de-energize the field, and this
will depend, other things being equal, upon the direction of the lines
of subdivision. If the subdivision be as indicated by the full lines in
Fig. 295, it is evident that if the current is of the same direction as
before, that is, from centre to periphery, its effect will be to
strengthen the field magnet; Whereas, if the subdivision be as indicated
by the dotted lines, the current generated will tend to weaken the
magnet. In the former case the machine will be capable of exciting
itself when the disc is rotated in the direction of arrow D; in the
latter case the direction of rotation must be reversed. Two such discs
may be combined, however, as indicated, the two discs rotating in
opposite fields, and in the same or opposite direction.
[Illustration: FIG. 295.]
[Illustration: FIG. 296.]
Similar disposition may, of course, be made in a type of machine in
which, instead of a disc, a cylinder is rotated. In such unipolar
machines, in the manner indicated, the usual field coils and poles may
be omitted and the machine may be made to consist only of a cylinder or
of two discs enveloped by a metal casting.
Instead of subdividing the disc or cylinder spirally, as indicated in
Fig. 295, it is more convenient to interpose one or more turns between
the disc and the contact ring on the periphery, as illustrated in Fig.
296.
A Forbes dynamo may, for instance, be excited in such a manner. In the
experience of the writer it has been found that instead of taking the
current from two such discs by sliding contacts, as usual, a flexible
conducting belt may be employed to advantage. The discs are in such case
provided with large flanges, affording a very great contact surface. The
belt should be made to bear on the flanges with spring pressure to take
up the expansion. Several machines with belt contact were constructed by
the writer two years ago, and worked satisfactorily; but for want of
time the work in that direction has been temporarily suspended. A number
of features pointed out above have also been used by the writer in
connection with some types of alternating current motors.
PART IV.
APPENDIX.--EARLY PHASE MOTORS AND THE TESLA MECHANICAL AND ELECTRICAL
OSCILLATOR.
CHAPTER XLII.
MR. TESLA'S PERSONAL EXHIBIT AT THE WORLD'S FAIR.
While the exhibits of firms engaged in the manufacture of electrical
apparatus of every description at the Chicago World's Fair, afforded the
visitor ample opportunity for gaining an excellent knowledge of the
state of the art, there were also numbers of exhibits which brought out
in strong relief the work of the individual inventor, which lies at the
foundation of much, if not all, industrial or mechanical achievement.
Prominent among such personal exhibits was that of Mr. Tesla, whose
apparatus occupied part of the space of the Westinghouse Company, in
Electricity Building.
This apparatus represented the results of work and thought covering a
period of ten years. It embraced a large number of different alternating
motors and Mr. Tesla's earlier high frequency apparatus. The motor
exhibit consisted of a variety of fields and armatures for two, three
and multiphase circuits, and gave a fair idea of the gradual evolution
of the fundamental idea of the rotating magnetic field. The high
frequency exhibit included Mr. Tesla's earlier machines and disruptive
discharge coils and high frequency transformers, which he used in his
investigations and some of which are referred to in his papers printed
in this volume.
Fig. 297 shows a view of part of the exhibits containing the motor
apparatus. Among these is shown at A a large ring intended to exhibit
the phenomena of the rotating magnetic field. The field produced was
very powerful and exhibited striking effects, revolving copper balls and
eggs and bodies of various shapes at considerable distances and at great
speeds. This ring was wound for two-phase circuits, and the winding was
so distributed that a practically uniform field was obtained. This ring
was prepared for Mr. Tesla's exhibit by Mr. C. F. Scott, electrician of
the Westinghouse Electric and Manufacturing Company.
[Illustration: FIG. 297.]
A smaller ring, shown at B, was arranged like the one exhibited at A but
designed especially to exhibit the rotation of an armature in a rotating
field. In connection with these two rings there was an interesting
exhibit shown by Mr. Tesla which consisted of a magnet with a coil, the
magnet being arranged to rotate in bearings. With this magnet he first
demonstrated the identity between a rotating field and a rotating
magnet; the latter, when rotating, exhibited the same phenomena as the
rings when they were energized by currents of differing phase. Another
prominent exhibit was a model illustrated at C which is a two-phase
motor, as well as an induction motor and transformer. It consists of a
large outer ring of laminated iron wound with two superimposed,
separated windings which can be connected in a variety of ways. This is
one of the first models used by Mr. Tesla as an induction motor and
rotating transformer. The armature was either a steel or wrought iron
disc with a closed coil. When the motor was operated from a two phase
generator the windings were connected in two groups, as usual. When used
as an induction motor, the current induced in one of the windings of the
ring was passed through the other winding on the ring and so the motor
operated with only two wires. When used as a transformer the outer
winding served, for instance, as a secondary and the inner as a primary.
The model shown at D is one of the earliest rotating field motors,
consisting of a thin iron ring wound with two sets of coils and an
armature consisting of a series of steel discs partly cut away and
arranged on a small arbor.
At E is shown one of the first rotating field or induction motors used
for the regulation of an arc lamp and for other purposes. It comprises a
ring of discs with two sets of coils having different self-inductions,
one set being of German silver and the other of copper wire. The
armature is wound with two closed-circuited coils at right angles to
each other. To the armature shaft are fastened levers and other devices
to effect the regulation. At F is shown a model of a magnetic lag motor;
this embodies a casting with pole projections protruding from two coils
between which is arranged to rotate a smooth iron body. When an
alternating current is sent through the two coils the pole projections
of the field and armature within it are similarly magnetized, and upon
the cessation or reversal of the current the armature and field repel
each other and rotation is produced in this way. Another interesting
exhibit, shown at G, is an early model of a two field motor energized by
currents of different phase. There are two independent fields of
laminated iron joined by brass bolts; in each field is mounted an
armature, both armatures being on the same shaft. The armatures were
originally so arranged as to be placed in any position relatively to
each other, and the fields also were arranged to be connected in a
number of ways. The motor has served for the exhibition of a number of
features; among other things, it has been used as a dynamo for the
production of currents of any frequency between wide limits. In this
case the field, instead of being energized by direct current, was
energized by currents differing in phase, which produced a rotation of
the field; the armature was then rotated in the same or in opposite
direction to the movement of the field; and so any number of
alternations of the currents induced in the armature, from a small to a
high number, determined by the frequency of the energizing field coils
and the speed of the armature, was obtained.
[Illustration: FIG. 298.]
The models H, I, J, represent a variety of rotating field, synchronous
motors which are of special value in long distance transmission work.
The principle embodied in these motors was enunciated by Mr. Tesla in
his lecture before the American Institute of Electrical Engineers, in
May, 1888[17]. It involves the production of the rotating field in one
of the elements of the motor by currents differing in phase and
energizing the other element by direct currents. The armatures are of
the two and three phase type. K is a model of a motor shown in an
enlarged view in Fig. 298. This machine, together with that shown in
Fig. 299, was exhibited at the same lecture, in May, 1888. They were the
first rotating field motors which were independently tested, having for
that purpose been placed in the hands of Prof. Anthony in the winter of
1887-88. From these tests it was shown that the efficiency and output of
these motors was quite satisfactory in every respect.
[17] See Part I, Chap. III, page 9.
[Illustration: FIG. 299.]
It was intended to exhibit the model shown in Fig. 299, but it was
unavailable for that purpose owing to the fact that it was some time ago
handed over to the care of Prof. Ayrton in England. This model was
originally provided with twelve independent coils; this number, as Mr.
Tesla pointed out in his first lecture, being divisible by two and
three, was selected in order to make various connections for two and
three-phase operations, and during Mr. Tesla's experiments was used in
many ways with from two to six phases. The model, Fig. 298, consists of
a magnetic frame of laminated iron with four polar projections between
which an armature is supported on brass bolts passing through the frame.
A great variety of armatures was used in connection with these two and
other fields. Some of the armatures are shown in front on the table,
Fig. 297, and several are also shown enlarged in Figs. 300 to 310. An
interesting exhibit is that shown at L, Fig. 297. This is an armature of
hardened steel which was used in a demonstration before the Society of
Arts in Boston, by Prof. Anthony. Another curious exhibit is shown
enlarged in Fig. 301. This consists of thick discs of wrought iron
placed lengthwise, with a mass of copper cast around them. The discs
were arranged longitudinally to afford an easier starting by reason of
the induced current formed in the iron discs, which differed in phase
from those in the copper. This armature would start with a single
circuit and run in synchronism, and represents one of the earliest types
of such an armature. Fig. 305 is another striking exhibit. This is one
of the earliest types of an armature with holes beneath the periphery,
in which copper conductors are imbedded. The armature has eight closed
circuits and was used in many different ways. Fig. 304 is a type of
synchronous armature consisting of a block of soft steel wound with a
coil closed upon itself. This armature was used in connection with the
field shown in Fig. 298 and gave excellent results.
[Illustration: FIG. 300.]
[Illustration: FIG. 301.]
[Illustration: FIG. 302.]
[Illustration: FIG. 303.]
[Illustration: FIG. 304.]
[Illustration: FIG. 305.]
[Illustration: FIG. 306.]
[Illustration: FIG. 307.]
[Illustration: FIG. 308.]
[Illustration: FIG. 309.]
[Illustration: FIG. 310.]
Fig. 302 represents a synchronous armature with a large coil around a
body of iron. There is another very small coil at right angles to the
first. This small coil was used for the purpose of increasing the
starting torque and was found very effective in this connection. Figs.
306 and 308 show a favorite construction of armature; the iron body is
made up of two sets of discs cut away and placed at right angles to each
other, the interstices being wound with coils. The one shown in Fig. 308
is provided with an additional groove on each of the projections formed
by the discs, for the purpose of increasing the starting torque by a
wire wound in these projections. Fig. 307 is a form of armature
similarly constructed, but with four independent coils wound upon the
four projections. This armature was used to reduce the speed of the
motor with reference to that of the generator. Fig. 300 is still another
armature with a great number of independent circuits closed upon
themselves, so that all the dead points on the armature are done away
with, and the armature has a large starting torque. Fig. 303 is another
type of armature for a four-pole motor but with coils wound upon a
smooth surface. A number of these armatures have hollow shafts, as they
have been used in many ways. Figs. 309 and 310 represent armatures to
which either alternating or direct current was conveyed by means of
sliding rings. Fig. 309 consists of a soft iron body with a single coil
wound around it, the ends of the coil being connected to two sliding
rings to which, usually, direct current was conveyed. The armature shown
in Fig. 310 has three insulated rings on a shaft and was used in
connection with two or three phase circuits.
All these models shown represent early work, and the enlarged engravings
are made from photographs taken early in 1888. There is a great number
of other models which were exhibited, but which are not brought out
sharply in the engraving, Fig. 297. For example at M is a model of a
motor comprising an armature with a hollow shaft wound with two or three
coils for two or three-phase circuits; the armature was arranged to be
stationary and the generating circuits were connected directly to the
generator. Around the armature is arranged to rotate on its shaft a
casting forming six closed circuits. On the outside this casting was
turned smooth and the belt was placed on it for driving with any desired
appliance. This also is a very early model.
On the left side of the table there are seen a large variety of models,
N, O, P, etc., with fields of various shapes. Each of these models
involves some distinct idea and they all represent gradual development
chiefly interesting as showing Mr. Tesla's efforts to adapt his system
to the existing high frequencies.
On the right side of the table, at S, T, are shown, on separate
supports, larger and more perfected armatures of commercial motors, and
in the space around the table a variety of motors and generators
supplying currents to them was exhibited.
The high frequency exhibit embraced Mr. Tesla's first original apparatus
used in his investigations. There was exhibited a glass tube with one
layer of silk-covered wire wound at the top and a copper ribbon on the
inside. This was the first disruptive discharge coil constructed by him.
At U is shown the disruptive discharge coil exhibited by him in his
lecture before the American Institute of Electrical Engineers, in May,
1891.[18] At V and W are shown some of the first high frequency
transformers. A number of various fields and armatures of small models
of high frequency apparatus as shown at X and Y, and others not visible
in the picture, were exhibited. In the annexed space the dynamo then
used by Mr. Tesla at Columbia College was exhibited; also another form
of high frequency dynamo used.
[18] See Part II, Chap. XXVI., page 145.
[Illustration: FIG. 311.]
In this space also was arranged a battery of Leyden jars and his large
disruptive discharge coil which was used for exhibiting the light
phenomena in the adjoining dark room. The coil was operated at only a
small fraction of its capacity, as the necessary condensers and
transformers could not be had and as Mr. Tesla's stay was limited to one
week; notwithstanding, the phenomena were of a striking character. In
the room were arranged two large plates placed at a distance of about
eighteen feet from each other. Between them were placed two long tables
with all sorts of phosphorescent bulbs and tubes; many of these were
prepared with great care and marked legibly with the names which would
shine with phosphorescent glow. Among them were some with the names of
Helmholtz, Faraday, Maxwell, Henry, Franklin, etc. Mr. Tesla had also
not forgotten the greatest living poet of his own country, Zmaj Jovan;
two or three were prepared with inscriptions, like "Welcome,
Electricians," and produced a beautiful effect. Each represented some
phase of this work and stood for some individual experiment of
importance. Outside the room was the small battery seen in Fig. 311, for
the exhibition of some of the impedance and other phenomena of interest.
Thus, for instance, a thick copper bar bent in arched form was provided
with clamps for the attachment of lamps, and a number of lamps were kept
at incandescence on the bar; there was also a little motor shown on the
table operated by the disruptive discharge.
As will be remembered by those who visited the Exposition, the
Westinghouse Company made a line exhibit of the various commercial
motors of the Tesla system, while the twelve generators in Machinery
Hall were of the two-phase type constructed for distributing light and
power. Mr. Tesla, also exhibited some models of his oscillators.
CHAPTER XLIII.
THE TESLA MECHANICAL AND ELECTRICAL OSCILLATORS.
On the evening of Friday, August 25, 1893, Mr. Tesla delivered a lecture
on his mechanical and electrical oscillators, before the members of the
Electrical Congress, in the hall adjoining the Agricultural Building, at
the World's Fair, Chicago. Besides the apparatus in the room, he
employed an air compressor, which was driven by an electric motor.
Mr. Tesla was introduced by Dr. Elisha Gray, and began by stating that
the problem he had set out to solve was to construct, first, a mechanism
which would produce oscillations of a perfectly constant period
independent of the pressure of steam or air applied, within the widest
limits, and also independent of frictional losses and load. Secondly, to
produce electric currents of a perfectly constant period independently
of the working conditions, and to produce these currents with mechanism
which should be reliable and positive in its action without resorting to
spark gaps and breaks. This he successfully accomplished in his
apparatus, and with this apparatus, now, scientific men will be provided
with the necessaries for carrying on investigations with alternating
currents with great precision. These two inventions Mr. Tesla called,
quite appropriately, a mechanical and an electrical oscillator,
respectively.
The former is substantially constructed in the following way. There is a
piston in a cylinder made to reciprocate automatically by proper
dispositions of parts, similar to a reciprocating tool. Mr. Tesla
pointed out that he had done a great deal of work in perfecting his
apparatus so that it would work efficiently at such high frequency of
reciprocation as he contemplated, but he did not dwell on the many
difficulties encountered. He exhibited, however, the pieces of a steel
arbor which had been actually torn apart while vibrating against a
minute air cushion.
With the piston above referred to there is associated in one of his
models in an independent chamber an air spring, or dash pot, or else he
obtains the spring within the chambers of the oscillator itself. To
appreciate the beauty of this it is only necessary to say that in that
disposition, as he showed it, no matter what the rigidity of the spring
and no matter what the weight of the moving parts, in other words, no
matter what the period of vibrations, the vibrations of the spring are
always isochronous with the applied pressure. Owing to this, the results
obtained with these vibrations are truly wonderful. Mr. Tesla provides
for an air spring of tremendous rigidity, and he is enabled to vibrate
big weights at an enormous rate, considering the inertia, owing to the
recoil of the spring. Thus, for instance, in one of these experiments,
he vibrates a weight of approximately 20 pounds at the rate of about 80
per second and with a stroke of about 7/8 inch, but by shortening the
stroke the weight could be vibrated many hundred times, and has been, in
other experiments.
To start the vibrations, a powerful blow is struck, but the adjustment
can be so made that only a minute effort is required to start, and, even
without any special provision it will start by merely turning on the
pressure suddenly. The vibration being, of course, isochronous, any
change of pressure merely produces a shortening or lengthening of the
stroke. Mr. Tesla showed a number of very clear drawings, illustrating
the construction of the apparatus from which its working was plainly
discernible. Special provisions are made so as to equalize the pressure
within the dash pot and the outer atmosphere. For this purpose the
inside chambers of the dash pot are arranged to communicate with the
outer atmosphere so that no matter how the temperature of the enclosed
air might vary, it still retains the same mean density as the outer
atmosphere, and by this means a spring of constant rigidity is obtained.
Now, of course, the pressure of the atmosphere may vary, and this would
vary the rigidity of the spring, and consequently the period of
vibration, and this feature constitutes one of the great beauties of the
apparatus; for, as Mr. Tesla pointed out, this mechanical system acts
exactly like a string tightly stretched between two points, and with
fixed nodes, so that slight changes of the tension do not in the least
alter the period of oscillation.
The applications of such an apparatus are, of course, numerous and
obvious. The first is, of course, to produce electric currents, and by a
number of models and apparatus on the lecture platform, Mr. Tesla showed
how this could be carried out in practice by combining an electric
generator with his oscillator. He pointed out what conditions must be
observed in order that the period of vibration of the electrical system
might not disturb the mechanical oscillation in such a way as to alter
the periodicity, but merely to shorten the stroke. He combines a
condenser with a self-induction, and gives to the electrical system the
same period as that at which the machine itself oscillates, so that both
together then fall in step and electrical and mechanical resonance is
obtained, and maintained absolutely unvaried.
Next he showed a model of a motor with delicate wheelwork, which was
driven by these currents at a constant speed, no matter what the air
pressure applied was, so that this motor could be employed as a clock.
He also showed a clock so constructed that it could be attached to one
of the oscillators, and would keep absolutely correct time. Another
curious and interesting feature which Mr. Tesla pointed out was that,
instead of controlling the motion of the reciprocating piston by means
of a spring, so as to obtain isochronous vibration, he was actually able
to control the mechanical motion by the natural vibration of the
electro-magnetic system, and he said that the case was a very simple
one, and was quite analogous to that of a pendulum. Thus, supposing we
had a pendulum of great weight, preferably, which would be maintained in
vibration by force, periodically applied; now that force, no matter how
it might vary, although it would oscillate the pendulum, would have no
control over its period.
Mr. Tesla also described a very interesting phenomenon which he
illustrated by an experiment. By means of this new apparatus, he is able
to produce an alternating current in which the E. M. F. of the impulses
in one direction preponderates over that of those in the other, so that
there is produced the effect of a direct current. In fact he expressed
the hope that these currents would be capable of application in many
instances, serving as direct currents. The principle involved in this
preponderating E. M. F. he explains in this way: Suppose a conductor is
moved into the magnetic field and then suddenly withdrawn. If the
current is not retarded, then the work performed will be a mere
fractional one; but if the current is retarded, then the magnetic field
acts as a spring. Imagine that the motion of the conductor is arrested
by the current generated, and that at the instant when it stops to move
into the field, there is still the maximum current flowing in the
conductor; then this current will, according to Lenz's law, drive the
conductor out of the field again, and if the conductor has no
resistance, then it would leave the field with the velocity it entered
it. Now it is clear that if, instead of simply depending on the current
to drive the conductor out of the field, the mechanically applied force
is so timed that it helps the conductor to get out of the field, then it
might leave the field with higher velocity than it entered it, and thus
one impulse is made to preponderate in E. M. F. over the other.
With a current of this nature, Mr. Tesla energized magnets strongly, and
performed many interesting experiments bearing out the fact that one of
the current impulses preponderates. Among them was one in which he
attached to his oscillator a ring magnet with a small air gap between
the poles. This magnet was oscillated up and down 80 times a second. A
copper disc, when inserted within the air gap of the ring magnet, was
brought into rapid rotation. Mr. Tesla remarked that this experiment
also seemed to demonstrate that the lines of flow of current through a
metallic mass are disturbed by the presence of a magnet in a manner
quite independently of the so-called Hall effect. He showed also a very
interesting method of making a connection with the oscillating magnet.
This was accomplished by attaching to the magnet small insulated steel
rods, and connecting to these rods the ends of the energizing coil. As
the magnet was vibrated, stationary nodes were produced in the steel
rods, and at these points the terminals of a direct current source were
attached. Mr. Tesla also pointed out that one of the uses of currents,
such as those produced in his apparatus, would be to select any given
one of a number of devices connected to the same circuit by picking out
the vibration by resonance. There is indeed little doubt that with Mr.
Tesla's devices, harmonic and synchronous telegraphy will receive a
fresh impetus, and vast possibilities are again opened up.
Mr. Tesla was very much elated over his latest achievements, and said
that he hoped that in the hands of practical, as well as scientific men,
the devices described by him would yield important results. He laid
special stress on the facility now afforded for investigating the effect
of mechanical vibration in all directions, and also showed that he had
observed a number of facts in connection with iron cores.
[Illustration: FIG. 312.]
The engraving, Fig. 312, shows, in perspective, one of the forms of
apparatus used by Mr. Tesla in his earlier investigations in this field
of work, and its interior construction is made plain by the sectional
view shown in Fig. 313. It will be noted that the piston P is fitted
into the hollow of a cylinder C which is provided with channel ports
O O, and _I_, extending all around the inside surface. In this
particular apparatus there are two channels O O for the outlet of the
working fluid and one, _I_, for the inlet. The piston P is provided with
two slots S S' at a carefully determined distance, one from the other.
The tubes T T which are screwed into the holes drilled into the piston,
establish communication between the slots S S' and chambers on each side
of the piston, each of these chambers connecting with the slot which is
remote from it. The piston P is screwed tightly on a shaft A which
passes through fitting boxes at the end of the cylinder C. The boxes
project to a carefully determined distance into the hollow of the
cylinder C, thus determining the length of the stroke.
Surrounding the whole is a jacket J. This jacket acts chiefly to
diminish the sound produced by the oscillator and as a jacket when the
oscillator is driven by steam, in which case a somewhat different
arrangement of the magnets is employed. The apparatus here illustrated
was intended for demonstration purposes, air being used as most
convenient for this purpose.
A magnetic frame M M is fastened so as to closely surround the
oscillator and is provided with energizing coils which establish two
strong magnetic fields on opposite sides. The magnetic frame is made up
of thin sheet iron. In the intensely concentrated field thus produced,
there are arranged two pairs of coils H H supported in metallic frames
which are screwed on the shaft A of the piston and have additional
bearings in the boxes B B on each side. The whole is mounted on a
metallic base resting on two wooden blocks.
[Illustration: FIG. 313.]
The operation of the device is as follows: The working fluid being
admitted through an inlet pipe to the slot I and the piston being
supposed to be in the position indicated, it is sufficient, though not
necessary, to give a gentle tap on one of the shaft ends protruding
from the boxes B. Assume that the motion imparted be such as to move the
piston to the left (when looking at the diagram) then the air rushes
through the slot S' and tube T into the chamber to the left. The
pressure now drives the piston towards the right and, owing to its
inertia, it overshoots the position of equilibrium and allows the air to
rush through the slot S and tube T into the chamber to the right, while
the communication to the left hand chamber is cut off, the air of the
latter chamber escaping through the outlet O on the left. On the return
stroke a similar operation takes place on the right hand side. This
oscillation is maintained continuously and the apparatus performs
vibrations from a scarcely perceptible quiver amounting to no more than
1 of an inch, up to vibrations of a little over 3/8 of an inch,
according to the air pressure and load. It is indeed interesting to see
how an incandescent lamp is kept burning with the apparatus showing a
scarcely perceptible quiver.
To perfect the mechanical part of the apparatus so that oscillations are
maintained economically was one thing, and Mr. Tesla hinted in his
lecture at the great difficulties he had first encountered to accomplish
this. But to produce oscillations which would be of constant period was
another task of no mean proportions. As already pointed out, Mr. Tesla
obtains the constancy of period in three distinct ways. Thus, he
provides properly calculated chambers, as in the case illustrated, in
the oscillator itself; or he associates with the oscillator an air
spring of constant resilience. But the most interesting of all, perhaps,
is the maintenance of the constancy of oscillation by the reaction of
the electromagnetic part of the combination. Mr. Tesla winds his coils,
by preference, for high tension and associates with them a condenser,
making the natural period of the combination fairly approximating to the
average period at which the piston would oscillate without any
particular provision being made for the constancy of period under
varying pressure and load. As the piston with the coils is perfectly
free to move, it is extremely susceptible to the influence of the
natural vibration set up in the circuits of the coils H H. The
mechanical efficiency of the apparatus is very high owing to the fact
that friction is reduced to a minimum and the weights which are moved
are small; the output of the oscillator is therefore a very large one.
Theoretically considered, when the various advantages which Mr. Tesla
holds out are examined, it is surprising, considering the simplicity of
the arrangement, that nothing was done in this direction before. No
doubt many inventors, at one time or other, have entertained the idea of
generating currents by attaching a coil or a magnetic core to the piston
of a steam engine, or generating currents by the vibrations of a tuning
fork, or similar devices, but the disadvantages of such arrangements
from an engineering standpoint must be obvious. Mr. Tesla, however, in
the introductory remarks of his lecture, pointed out how by a series of
conclusions he was driven to take up this new line of work by the
necessity of producing currents of constant period and as a result of
his endeavors to maintain electrical oscillation in the most simple and
economical manner.
INDEX.
Alternate Current Electrostatic Apparatus 392
Alternating Current Generators for High Frequency 152, 374, 224
Alternating Motors and Transformers 7
American Institute Electrical Engineers Lecture 145
Anthony, W. A., Tests of Tesla Motors 8
Apparatus for Producing High Vacua 276
Arc Lighting, Tesla Direct, System 451
Auxiliary Brush Regulation 438
Biography, Tesla 4
Brush, Anti-Sparking 432
Brush, Third, Regulation 438
Brush, Phenomena in High Vacuum 226
Carborundum Button for Tesla Lamps 140, 253
Commutator, Anti-Sparking 432
Combination of Synchronizing and Torque Motor 95
Condensers with Plates in Oil 418
Conversion with Disruptive Discharge 193, 204, 303
Current or Dynamic Electricity Phenomena 327
Direct Current Arc Lighting 451
Dischargers, Forms of 305
Disruptive Discharge Coil 207, 221
Disruptive Discharge Phenomena 212
Dynamos, Improved Direct Current 448
Early Phase Motors 477
Effects with High Frequency and High Potential Currents 119
Electrical Congress Lecture, Chicago. 486
Electric Resonance 340
Electric Discharges in Vacuum Tubes 396
Electrolytic Registering Meter 420
Eye, Observations on the 294
Flames, Electrostatic, Non-Consuming 166, 272
Forbes Unipolar Generator 468, 474
Franklin Institute Lecture 294
Generators, Pyromagnetic 429
High Potential, High Frequency:
Brush Phenomena in High Vacuum 226
Carborundum Buttons 140, 253
Disruptive Discharge Phenomena 212
Flames, Electrostatic, Non-Consuming 166, 272
Impedance, Novel Phenomena 194, 338
Lighting Lamps Through Body 359
Luminous Effects with Gases 368
"Massage" with Currents 394
Motor with Single Wire 234, 330
"No Wire" Motors 235
Oil Insulation of Induction Coils 173, 221
Ozone, Production of 171
Phosphorescence 367
Physiological Effects 162, 394
Resonance 340
Spinning Filament 168
Streaming Discharges of High Tension Coil 155, 163
Telegraphy without Wires 346
Impedance, Novel Phenomena 194, 338
Improvements in Unipolar Generators 465
Improved Direct Current Dynamos and Motors 448
Induction Motors 92
Institution Electrical Engineers Lecture 198
Lamps and Motor operated on a Single Wire 330
Lamps with Single Straight Fiber 183
Lamps containing only a Gas 188
Lamps with Refractory Button 177, 239, 360
Lamps for Simple Phosphorescence 187, 282, 364
Lecture, Tesla before:
American Institute Electrical Engineers 145
Royal Institution 124
Institution Electrical Engineers 198
Franklin Institute and National Electric Light Association 294
Electrical Congress, Chicago 486
Lighting Lamps Through the Body 359
Light Phenomena with High Frequencies 349
Luminous Effects with Gases at Low-Pressure 368
"Magnetic Lag" Motor 67
"Massage" with Currents of High Frequency 394
Mechanical and Electrical Oscillators 486
Method of obtaining Direct from Alternating currents 409
Method of obtaining Difference of Phase by Magnetic Shielding 71
Motors:
With Circuits of Different Resistance 79
With Closed Conductors 9
Combination of Synchronizing and Torque 95
With Condenser in Armature Circuit 101
With Condenser in one of the Field Circuits 106
With Coinciding Maxima of Magnetic Effect in Armature and Field 83
With "Current Lag" Artificially Secured 58
Early Phase 477
With Equal Magnetic Energies in Field and Armature 81
Or Generator, obtaining Desired Speed of 36
Improved Direct Current 448
Induction 92
"Magnetic Lag" 67
"No Wire" 235
With Phase Difference in Magnetization of Inner and Outer Parts
of Core 88
Regulator for Rotary Current 45
Single Circuit, Self-starting Synchronizing 50
Single Phase 76
With Single Wire to Generator 234, 330
Synchronizing 9
Thermo-Magnetic 424
Utilizing Continuous Current Generators 31
National Electric Light Association Lecture 294
"No Wire" Motor 235
Observations on the Eye 294
Oil, Condensers with Plates in 418
Oil Insulation of Induction Coils 173, 221
Oscillators, Mechanical and Electrical 486
Ozone, Production of 171
Phenomena Produced by Electrostatic Force 318
Phosphorescence and Sulphide of Zinc 367
Physiological Effects of High Frequency 162, 394
Polyphase Systems 26
Polyphase Transformer 109
Pyromagnetic Generators 429
Regulator for Rotary Current Motors 45
Resonance, Electric, Phenomena of 340
"Resultant Attraction" 7
Rotating Field Transformers 9
Rotating Magnetic Field 9
Royal Institution Lecture 124
Scope of Lectures 119
Single Phase Motor 76
Single Circuit, Self-Starting Synchronizing Motors 50
Spinning Filament Effects 168
Streaming Discharges of High Tension Coil 155, 163
Synchronizing Motors 9
Telegraphy without Wires 246
Transformer with Shield between Primary and Secondary 113
Thermo-Magnetic Motors 424
Thomson, J. J., on Vacuum Tubes 397, 402, 406
Thomson, Sir W., Current Accumulator 471
Transformers:
Alternating 7
Magnetic Shield 113
Polyphase 109
Rotating Field 9
Tubes:
Coated with Yttria, etc. 187
Coated with Sulphide of Zinc, etc. 290, 367
Unipolar Generators 465
Unipolar Generator, Forbes 468, 474
Yttria, Coated Tubes 187
Zinc, Tubes Coated with Sulphide of 367