Improvements in Oscillographs or Apparatus for Indicating, or Recording, Rapidly Varying Electric Currents, or Potential Differences.
Year: 1898

page_text: Date of Application, 5th Mar., 1898-Accepted, 30th Apr., 1898
COMPLETE SPECIFICATION.
Improvements in Oscillographs or Apparatus for Indicating, or
Recording, Rapidly Varying Electric Currents, or Potential
Differences.
I, WILLIAM DUBOIS DUDDELL, of 47, Hans Place, Chelsea, in the County of
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 ascer-
tained in and by the following statement: -
The present invention relates to improvements in "oscillographs," or apparatus
for indicating, or recording, rapidly varying electric currents, or potential differ-
ences, especially applicable to alternate current wave forms.
The accuracy obtainable with an apparatus of this kind is largely dependent
on the following four conditions:-
1. The vibrating system must have a very small free periodic time.
2. The damping of the system must be sufficient and easily adjustable to the
correct value.
3. The sensibility must be sufficient.
4. A convenient method of observing and recording the results must be provided.
My invention relates to the details of the apparatus which have enabled me to
more closely comply with the above four conditions than has heretofore been done.
My apparatus is based on the well-known fact that if two flexible conductors
carrying a current in opposite directions be fixed at both ends and stretched across
a magnetic field, they will tend to bend in opposite directions and will turn a
mirror fixed to both of them. If the angle of rotation is small it will be very
approximately proportional to the current flowing in the conductors.
In my apparatus I preferably employ as conductors a metal strip, many times
wider than it is thick, bent back on itself over a pulley attached to a spring, or
a spring balance, and placed in a strong magnetic field produced by an electro-
magnet or other suitable means.
In the following Specification, this strip bent back on itself will be referred
to as a "pair of strips." For some purposes, as for example, when the simultaneous
observations of current and potential difference waves are required, it is necessary
to use a double instrument with two pairs of strips, or if more than two simul-
taneous observations are required an instrument having the requisite number of
pairs of strips may be used.
1. I will now proceed to describe my improvements by reference to the accom-
panying drawings. Fig. 1 is an elevation of the poles of a magnet shewing a
pair of strips and mirror in place, but with the front plate and window removed.
Fig. 2 is a horizontal section on the line a a¹. Figs. 3 and 4 are respectively
section and plan of tangent wheel adjusting heads for a double instrument. In
these figures N, and S, are the poles or pole pieces of the magnet. s,s are the pair
of strips formed by bending a strip back on itself over the pulley P, attached to
a spring, or spring balance, not shewn. To limit the free vibrating length of the
strips they are clamped on the block K, of insulating material and pass over the

page_text: guide block or pulley G, Fig. 1. The mirror M, is attached near the centres of
the vibrating portions by means of shellac varnish or by any of the ordinary
methods, care being taken that neither the mirror nor its mode of attachment
shall short circuit the strips.
2. To decrease the free periodic time, I generally make the length of the vibrating
part of the strips very short, preferably less than 5 centimetres, though I do not
limit myself to this length, and I stretch them by means of a spring, or spring
balance, as tight as they will bear without danger of breaking. I also employ a
mirror having a very small moment of inertia.
3. The damping is effected by the fluid friction produced by a transparent,
viscous, insulating liquid in which the vibrating portions of the strips, and the
mirror attached, are immersed and move. This liquid is contained in a chamber.
formed by closing the gap between the poles N, S, of the magnet front and back,
by means of blocks L, L', Fig. 2, formed of some non-magnetic substance.. A
window is inserted in the front block to allow light to reach the mirror.
It is necessary, however, in order to make the arrangement effective, that the
strips should be wider than can be conveniently rolled or that the clearance spaces
round them should be very small. Therefore, I prefer to insert a partition P,
Figs. 1 and 2, preferably of soft iron or steel which is partly cut away to allow
room for the mirror. This partition virtually causes each strip to move in its
own chamber, and by this means so increases the damping that it is quite easy to
obtain sufficient with convenient sized strips and clearance spaces. I adjust the
damping by varying the liquid used or by altering its temperature, or by both.
I would here point out that in order to make the drawings clear the clearance
spaces round the strips in Figs. 1 and 2 are shown much larger than in the actual
instrument.
4. To obtain sufficient sensibility: First, I place the strips close together, which
can be easily done. by having the above partition 2, Figs. 1 and 2 very thin.
Second, I shape the poles or pole picces in such a way as to concentrate the lines
of force on the strips as shewn in Fig. 2. And third, I employ a very narrow
gap between the poles. There is a risk, however, when using an electromagnet,
of the poles N, S, tending to move each time the exciting current is made, or
broken and thus causing the joints between them and the blocks L, L', to leak
and allow the damping fluid to escape. Instead therefore of fastening the pole-
pieces N, S, to the magnet, I generally attach them to the blocks L, L', and so
build up the pole pieces and chambers for the damping liquid as one whole as
shewn in Figs. 1 and 2. And this piece N, L, S, L', inserted as a whole between
the ends of the magnet core S¹, N¹, a slight amount of clearance being left
between N, and S', and between S, and N¹. Or I may form the chambers which
hold the liquid quite separately from the poles of the magnet.
5. To observe or record the motion of the mirror M, Fig. 1, a source of light
and a lens system are used in the ordinary way, and the movement of the spot
of light may be photographed on a moving plate or film, or observed in a rotating
mirror. In my improved method of observing and tracing the curve when working
on alternate current wave forms, I employ a synchronous alternate current motor
driven from the circuit which is being investigated. To the axle of this motor
is connected the rotating mirror either direct or by means of gearing and thus the
mirror moves synchronously with the current 'whose wave form it is desired to record.
The beam of light from the oscillograph falls on this synchronous mirror and is
reflected from it on to an observing screen which I find it desirable to put close to
the rotating mirror, and, therefore, to have semitransparent. The wave form
will appear at rest on the screen and so can be accurately drawn or photographed.
6. It is necessary that some method of setting the zero should be provided
especially when two or more simultaneous records are required, it is also convenient
in a multiple instrument to place the several pairs of strips close together in the 5
magnetic circuit in order that one lens system and source of light may be used.

page_text: To enable me to set the zeros and at the same time have the pairs of strips close
together, I employ in an instrument having two pairs of strips the tangent heads
shewn in Figs. 3 and 4.
Fig. 4 is a plan of a pair of tangent heads from the right hand side of which
the tangent screw, its bearing and the tangent plate are removed. Fig. 3 is a
section through one head on the line b, 61. In these figures to avoid confusion
the strips are omitted. In these figures T, is a tangent screw which engages with
a tangent plate A, which is fixed to the block B. The block B turns in the plate C,
and carries the guide pulley G, over which the strips pass. If the tangent screw T,
is turned it rotates A, and the block B, thus turning the guide pulley G, and
rotating the strips and mirror about a vertical axis giving the necessary horizontal
adjustment. To provide the vertical adjustment, I preferably make each head
complete in itself and attach it to the pole pieces N, S, or frame of the instrument,
either by means of screws and slotted holes as shewn at H, or by means of a screw
and slide as in the slide rest of a lathe. So that the heads can be moved back-
wards and forwards and give the vertical adjustment to the spots. Now in the
Figs. 3 and 4, the vibrating systems are very close together, but in order to attain
this nearly half the block B, and the hole in the plate C, in which it turns have
had to be cut away and from the plan Fig. 3 it is evident that the pressure of the
tangent screw T, would cause the block B, to jam in its hole, but to prevent this
I turn a kind of dovetail' or equivalent device, shewn at D, Fig. 3, on the block B
and a corresponding recess in the plate C, to receive it, this dovetail or projection
takes the side thrust of the tangent screw and effectually prevents the block B,
from jamming.
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 use of a vibrating system consisting of a flat strip many times wider
than it is thick.bent back on itself and stretched tight by means of a spring, or
spring balance, and placed in a strong magnetic field and immersed in a viscous
damping liquid, its movement being observed and recorded by means of a mirror
attached to it substantially as hereinbefore described.
(2) The use of a partition preferably of soft iron or steel between a pair of strips
to enable sufficient damping to be obtained as described in paragraph (3) and
illustrated in Figs. 1 and 2.
(3) The use of tangent heads constructed as described in paragraph (6) and
illustrated in Figs. 3 and 4, for an instrument having more than one vibrating
system.
(4) The use of detached piece N, L, S, L¹, described in paragraph (4) and
illustrated in Figs. 1 and 2.
(5) The use of a synchronous motor to drive the rotating mirror in combination
with the oscillograph as described in paragraph (5).
Dthin the will
С
TL
1
WILLIAM DU BOIS DUDDELL.