Improvements in Electric Transformers.
Year: 1896

page_text: Improvements in Electric Transformers.
LESLIE BRADLEY MILLER of 2 Gray's Inn Road, London Electrical Engineer
do hereby declare the nature of this invention to be as follows:-
It relates to what are commonly called Tesla transformers. The chief object is
the construction of a compact apparatus the insulation of which cannot break down
permanently, and which has no parts easily injured by improper usage.
As a safeguard against overheating I prefer to employ a step-up transformer
with the primary outside and the secondary inside an oil bath, the two coils being
on opposite sides of a vertical iron rectangle. The primary winding is sub-divided
so as to alter the ratio of conversion. This transformer charges a condenser
made of sheet glass and metal foil. The Tesla transformer has the secondary
wound on an insulating tube, and the winding is separated from the primary
winding, or from other layers of the secondary by longitudinal rods of ebonite
acting as distance pieces. To facilitate the winding these rods may be covered
by a sheet of some thin insulating material when necessary but a narrow longitudinal
gap should be left at the top in order that air bubbles may be able to escape.
The insulation therefore depends on the oil. After one layer of secondary has
been wound the end may be taken through inside the tube and the winding
recommend over the beginning in order to halve the tension between the two ends
or the winding may be carried out in two four or more sections with an oil space
between them. The primary winding may be thick bare copper first wound into
a spiral and then slipped on the ebonite rods which can be grooved to keep the
turns at a proper distance apart. The whole coil is held together between two
end plates and an ebonite plate can also be fixed in the middle to support the
reds. The spark gap is adjustable, the ends are tipped with zinc and the rods
carrying them made to work inside a sound deadening box. The condenser is
preferably placed vertically inside the box and has its two electrodes attached
permanently to the two ends of the primary winding under the surface of the oil.
The spark gap is interposed half way in the primary the wires being brought up
through the cover of the box on the top of which it is mounted at one side, with
suitable terminals for attaching the step up transformer. On the other side
of the box adjustable discharge rods for the secondary winding are fixed at
opposite corners.
If solid paraffin be used as the insulator instead of oil the general arrangement
of the parts remains the same.
Dated this 11th day of June 1896.

page_text: LESLIE BRADLEY MILLER of 2 Gray's Inn Road London Electrical Engineer
do hereby declare the nature of this invention and in what manner the same is
to be performed, to be particularly described and ascertained in and by the
following statement :-
My invention relates to improvements in electrical transformers for giving very
high tensions, and my aim is the construction of a compact apparatus, the
insulation of which is not liable to break down in such a way as to cause expensive
repairs to be necessary.
The chief purpose for which I propose to utilize the appliances after described,
are the working of vacuum tubes of all kinds, ozone generators, for cable testing,
and for use with any kind of apparatus more or less of the nature of an electrical
condenser. Also for electro therapeutical purposes.
In order to raise the voltage of an alternating current from hundreds of volts
to thousands, I make use of the direct step-up transformer afterwards described
and to raise the pressure still higher a modification of a "Tesla" transformer, i.e. one
in which the oscillating discharge of a condenser is utilized to produce rushes of
current through a primary winding.
The voltage of the step-up transformer if used without the Tesla coil may
also be raised by placing a condenser of suitable capacity in parallel with the
secondary or by allowing a discharge to take place across an air gap in parallel
with the secondary, or by a combination of the two means. The rate of discharge
may be regulated by another adjustable condenser in series with the secondary
winding. An excessive output, and consequent heating, will however be prevented
in the step-up transformer after described, by reason of the large magnetic
leakage when the primary is wholly wound on one limb and the secondary on
the other.
The step-up transformer I prefer to employ has the primary winding outside
and the secondary inside an oil bath, the two coils being on opposite sides of an
iron rectangle placed vertically in a suitable containing box. This rectangle is
made by bending laminated strip iron into the shape of the letter U, with the
parallel sides horizontal, then slipping the primary coil on the upper limb and
the secondary coils on the lower limb, and then bending the free ends of the
iron strip round one by one to complete the magnetic circuit; or the iron
circuit may be made by clamping four bundles of iron together at the four
corners.
The secondary winding of this transformer is carried out in the manner usually
adopted for induction coils wound in two sections, but it may also be wound in
one section or in many sections, so that the tension increases from the first to
the last.
The primary winding is sub-divided so that the ratio of conversion can be altered,
the ends of the loops brought out being brought to suitable terminals or to the
contacts on a multiple switch of the nou-short-circuiting, non-contact-breaking
type, well known as an accumulator switch. The contacts on the switch, must
however be well protected as under certain conditions a dangerous difference of
potential may be set up between some of them.
Part of the primary winding may also be under or over the secondary winding,
so as to prevent magnetic leakage of lines and the same method of insulation, by
means of longitudinal rods, adopted as in the Tesla coil after described.
Instead of altering the ratio of the turns on the high tension transformer there
may be only one winding on it and the excitation can be varied by using a separate

page_text: transformer for exciting it with a sub-divided secondary and a switch for adding
the volts of the primary to the secondary, if a greater range than the secondary
alone can give is required.
This transformer charges the condenser which is made of sheet glass and copper
or tin foil in the usual manner. As it is of great importance to get rid of the
air between the plates, I prefer to place the condenser vertically in the oil, so
that the air may easily be able to escape and be replaced by a thin layer of oil."
The Tesla transformer in its simplest form is constructed as follows:-
Several hundred turns of wire covered with one or more layers of cotton are
wound on an insulating tube; on this three or more sticks of ebonite are laid
longitudinally to act as distance pieces between the primary and secondary; the
rods are grooved at suitable intervals and the primary winding consisting of a few
turns of thick bare copper is wound on the ebonite, being held in position by the
grooves. The cotton on the primary winding serves only to keep the copper a
convenient distance apart, so that the insulation between turn and tarn depends
wholly on the oil between them, as does that between the primary and secondary.
If more than one layer of primary winding is employed, longitudinal rods of ebonite
are placed between the layers and these can be advantageously placed at an angle
to the rods underneath, so that the distance between the layers may be a minimum
where there is no tension and a maximum at the end where the tension is greatest.
Or the secondary can be wound in sections with suitable oil spaces between the
sections, and the primary separated from the secondary by ebonite or glass rods as
before described.
Where it is convenient to use a tube of some kind to wind
a gap should be left at the top, so that air can readily escape.
also be placed inside the secondary instead of outside.
upon, in all cases
The primary may
My object is to depend wholly on oil for the insulating medium, to allow air to
escape readily and to avoid the employment of various insulators liable to be
pierced by a spark or to heat from static hysteresis.
The longitudinal rods are held in position by ebonite end plates and a plate
can also be placed in the middle. The spark gap can be placed anywhere in the
primary circuit. The spark takes place between the ends of two screws tipped
with zinc and the sparking contact is enclosed in a transparent box in order to
deaden the sound.
If the step-up transformer is to be used for working the Tesla the whole can be
enclosed in one box and covered with oil, but I prefer to place the step-up trans-
former in one box and the condenser and Tesla coil in another.
The Tesla box may consist of an earthenware trough contained in an outer shell
of polished wood; this has a top which has a longitudinal opening fitted with a lid,
the spark gap is mounted on one side of the top of the box and pillars of ebonite
on the other to carry the discharge rods. The ebonite pillars pass right through
the top of the box and below the surface of the oil. A metal core through them is
in communication with the end of the secondary and with suitable discharge
terminals at the top of the pillars. Instead of using oil a mixture of oil and resin
or paraffin wax may be poured in hot and allowed to set afterwards.
Referring to the drawings
Fig. 1 is a longitudinal section through the step-ap transformer. p is the
primary winding and the secondary. The dotted line above the secondary
winding shows the level of the insulating oil.
Fig. 2 is a section through A. A. showing the connection to the high tension
terminal through an ebonite pillar, and also one of the low tension terminals l.
Fig. 3 is a diagram showing the methods by which the voltage of the step-up
transformer may be raised and the current through each of various apparatus
worked from it controlled. p is the primary circuit having loops brought out so
that the ratio between the primary and secondary may be varied by means of a
switch as described. C is a condenser of suitable capacity in parallel with the
secondary winding and G a spark gap. X. X. are X-ray tubes, or similar

page_text: appliances, the current through each of which is regulated by means of its own
adjustable condenser in series with it.
Fig. 4 is a section through the Tesla transformer e, e, being the ebonite rods on
which the primary P is wound the diameter of these rods being such that the
primary winding is kept sufficiently far off the secondary S for the layer of oil to
prevent sparking between the primary and secondary. The ends of the secondary
are led to pins fixed in the ebonite at s, s, and from there through ebonite rods to
the secondary terminals.
Fig. 5 shows a section through B B and also the vertical condenser C with the
connecting wire from it to the primary. The condenser is made with sheets of
glass g, or mica, with copper foil m between each. All along the horizontal top of
this condenser there is nothing to prevent the air confined between the plates, when
the oil is first poured in, from gradually rising upward and bubbling through the
cil above, being replaced by oil. The sheets of copper forming one coat of the
condenser are soldered together at the vertical side and a thick piece of brass b
fixed to them carrying a screw at the top for connecting the primary P. At the
other end they are also soldered together and a wire taken to one side of the
spark gap.
Fig. 6 shows in the lower box the step-up transformer with wires from its chigh
tension terminals connected to two terminals under the spark gap S G of the Tesla
transformer shown in the larger box. These two terminals are connected to the
two poles of the condenser which stands vertically in the large box between the
coil and the side of box where the spark gap stands. It is hidden from view in
the drawing. In Fig. 6 the primary winding is shown divided into two parts by
an ebonite disc the spark gap being interposed in the middle of the primary winding
instead of at the end.
Fig. 7 shows, diagrammatically, the various circuits.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed I declare that what I
claim is :-
(1.) The step-up transformer substantially as described and used in the manner
and for the purposes above mentioned.
(2.) A "Tesla" coil constructed and insulated as described..
(3.) The combination of a Tesla coil with a vertical condenser as described.
(4.) The combination of a step-up transformer, Tesla coil, spark gap, and
condenser as described.
Dated this 9th day of March 1897.
LESLIE B. MILLER.