AN IMPROVED INSTRUMENT FOR MEASURING ANGLES
Year: 1885

page_text: PERCY NEVILL Harbour Works Douglas I. O. Man Civil Engineer do hereby
declare the nature of my invention for AN IMPROVED INSTRUMENT FOR MEASURING
ANGLES to be as follows:-
A Bar centred at one end, has a roller at the other.
When the bar is swung round on the centre, the roller revolves.
The distance between the Roller and the Centre is so adjusted, that the revolu-
tions & fractions of revolutions of the roller are made to indicate angles passed
through by the bar.
On the axle of the roller is a worm gearing into a pinion attached to a disc.
The Disc & Roller are divided and figured so as to show the angle passed through
by the bar in degrees & minor fractions of arc.
For fine reading a Vernier is attached.
The Roller is made so that it shall not slip upon the supporting surface.
Besides the centre & the roller, the bar has a third point of support, not in a
line with the other two, so that the plane of these three points is constantly' at the
same angle to the supporting surface.
The sizes of the roller, the divisions on the roller, that part of the roller which
is divided, the divisions on the Vernier, the disc, & the divisions on the disc, as
well as the length of the bar, may all be unknown quantities to the designer &
maker of the instrument as it measures not lengths but angles.

page_text: Improved Instrument for measuring Angies.
PERCY NEVILL Harbour Works Douglas Isle of Man Civil Engineer I do hereby
declare the nature of my invention for IMPROVED INSTRUMENT FOR MEASURING
ANGLES and in what manner the same is to be performed, to be particularly
described and ascertained in and by the following statement :—
A straight-edge, made so that it will turn about a centre near one end, has a
roller at any convenient distance from that centre.
The axis of the Roller is in alignment with that centre.
On the axle of the Roller is a worm gearing into a toothed wheel which supports
and is fixed to a Dial plate.
When the straight-edge is swung round on the centre, the Roller, bearing on a
supporting surface, revolves.
The distance between the roller & the centre is so adjusted that the revolutions
& fractions of revolutions of the roller are made to indicate angles passed through
by the straight-edge when the latter is swung on the centre.
The Dial & roller are divided & figured to give the angle passed through by the
straight-edge in degrees & minor fractions of arc, & for fine reading a Vernier is
attached.
The Roller is made so that it shall not slip upon the supporting surface.
Besides the centre & the Roller, the straight-edge has a third point of support
not in the line of the other two, so that the plane of these three points is con-
stantly at the same angle to the supporting surface.
For giving an even & steady pressure upon the instrument a spring carrying a
small roller is attached, so that when the spring is depressed & the small roller
brought to bear upon the supporting surface, increase or diminution of pressure on
the top of the spring, & over the small roller, does not affect the pressure of the
three points of support of the straight-edge upon the surface against which they,
rest.
Parallel & near to the side of the straight-edge is a slide, & on this fits a saddle
which can be slid along the length of the straight-edge. The saddle carries a
needle-point &ot for marking the angles measured.
The end of the straight-edge is made square so as to give the direction of the
tangent of the angle measured.
The centre of rotation of the straight-edge is a "needle-point" recessed half its
thickness into the working side of the straight-edge & so attached that the view
of it is not obscured by its supports.
The Bearings of the main roller axis have centres which are adjustable in the
direction of the centre line of the instrument
To prevent the roller revolving when adding cumulative angles & shifting the
instrument, a Brake is attached consisting of a spring which is pushed against the
periphery of the roller by means of a cam lever, in such manner that the brake
must be either 'on' or 'off' & cannot be partly on, i.e. the instrument is either
free or absolutely locked.
After the position of the Centre has been accurately fixed, the straight-edge if
required, may be divided to soine scale.

page_text: H-Slide.
As will be seen from the Drawing the roller projects below the plate, the latter
having a slot to receive it. The Roller consists of three parts (a) a thick round-
edged disc; (b) a wheel having sharp teeth; & (c) a divided drum. The Roller,
i.e. the drum portion of it, is divided into ten equal & numbered parts which are
single degrees of the arc of which the distance N-R is a radius. They are
made to equal exactly the required fraction of arc by increasing or diminishing
the radius N-R by means of the adjustable centres.
These divisions are sub-divided into six, which therefore are each equal to ten
minutes of the same arc.
The Vernier reads to tenths of these ten-minute sub-divisions i.c. to single
minutes.
The worm on the roller axle gears with a toothed wheel of thirty six teeth so
that ono revolution of the roller turns the toothed wheel through one thirty-sixth
of a complete revolution. The Dial being fixed to the toothed wheel is likewise
turned one thirty-sixth of a revolution & as it is divided into thirty-six equal
radial spaces, one of these must pass the arrow on the index post for each complete
revolution of the roller ten degrees.
=
The divisions on the Dial being numbered 10, 20, 30, 40, & so on, & the divisions
on the roller being numbered 1, 2, 3, 4, & so on, by simply adding the two readings,
the number of degrees passed through by the straight-edge, when rotated about its
centre, is at once obtained, the minutes being read by the Vernier.
One object of the design shown in the drawing is to have a small, portable, &
convenient instrument for measuring angles equivalent in accuracy, & having an
equal radius, to a costly & cumbersome circular protractor or station-pointer.
For measuring angles on globes, the straight-edge would be straight in one plane
only.
It can also be used for measuring angles in the field as it does not require a large
plane surface to work upon & may be used when the more elaborate but less
portable theodolite is unobtainable; & the angles taken may be plotted with the
same instrument.
Should the needle-point centre be broken, a new one can readily be substituted.
The needle is cut off flush with the top of the cap shown in the drawing, so when
that is removed there is a short length of needle sticking up which enables the
whole needle to be withdrawn & a new one put in.
An inner row of figures on the Dial & Roller enables readings to be taken back-
wards or forwards.

page_text: Backlash or shake between the worm & the toothed wheel, within wide limits,
does not affect the absolute exactitude of the [readings as all angles under ten
degrees show on the Roller & are read thereon only & the relative positions of
roller & dial at once show the correct reading.
The object of the compound roller before described is to get entire freedom from
slip & yet not to allow the sharp teeth, previously mentioned, to penetrate, or make
unsightly marks on the supporting surface.
The equalising spring S on drawing enables the instrument to give exact readings
in spite of great variation in the pressure applied to move it.
The straight-edge being raised above the supporting surface by the thickness of
the rubbing pieces it enables a line to be drawn, with the pen or otherwise, from
the needle-point centre the whole length of the bar & immediately beneath its
edge.