The Director is an instrument for measuring horizontal and vertical angles.
The No.7 director is similar in design and operation to the No.6a director.
The distinguishing features of the No.7 director are telescope and elevating gear which are designed to enable angles of sight from 65 degrees elevation to 15 degrees depsression to be read.
Each director has a registered number which is stamped on the side of the body with maker's name and date of manufacture. The main parts of the director are described in the following paragraphs and illustrated in the plates.
(a) The base- In the bottom of the base is a screw-threathed hole to take the clamping screw of the stand, there are also three grooves which fit over studs on the stand.
(b) The horizontal scale- This is pivoted on the base and is rotated in it by means of the lower gear. The scale is graduated every degree and is figured every 10 degrees from 0 degrees to 360 degrees.
(c) The lower gear- This concsists of a quick release lever and a slow motion spindle firred with a fluted head at each end.
When the quick release lever is pressed down, the slow motion spindle is disengaged and the director can be turned rapidly on the base.
when the lower gear is used, the horizontal scale, carrying with it the body and telescope, ism oved without changing the reading of the scale.
(d) The body- The body is pivoted on the horizontal scale and is rotated on it by means of the upper gear. A black pointer is formed on the body under the eyepiece of the telescope and is used to read the degrees of the horizontal scale.
There is also a red pointer on the body under the object glass of the telescope which is used o read the degrees of the horizontal scale in certain circumstances.
(e) The upper gear- This consists of a quick release lever and a slow montion spindle.
The slow montion spindle has a miled head on one end, and on the other end a micrometer drum held in place by a fixing screw.
The upper quick release lever and head are milled in order that they may be distinguished and the body can be turned rapidly on the horizontal scale.
When the upper gear is used, the body with telescope is moved and the reading of the horizontal scale is changed.
One turn of the spindle moves the director through two degrees.
The micrometer scale is graduated every two minutes and figured every 10 minutes on a conical drum.
The drum is read against a pointer on the body and is used in conjunction with the horizontal degree scale.
(f) The compass- Inside the top off the body is a compass needle. The needle is normally kept off its pivot and clamped by a spring plunger.
The needle is provided with a metal rider for adjusting for dip.
When the compass is to be used, the plunger is pressed in, this lowers the needle on to its pivot, on which is then free to swing.
The south end of the compass is marked S, and is read against an arrow, which is the laying mark, the position of which can be altered which is the laying mark, the position of which can be altered from its central position to any setting from 30 degrees east of 30 degrees west.
The setting is carried out by removing the small square headed cap adjusting compass situated below the rain shade of the object glass inverting this cap, inserting the square end in the socket and twisting until the magentic variation scale is at the setting required. The moveable portion is a verniet graduated to read to 10 minutes, there being six graduations east and six graduations west.
(g) Bubbles- A circular spirit (eight minute) bubble is sitauted on the front of the body for use in levelling the instrument.
(h) Telescope- The telescope, elevating arc, open sight and levelling (30 second) bubble are carried on trunnions on the body.
The optical system is so arranged that the eyepiece is inclined at an angle of 50 degrees to the line of sight from the object to the object glass. This arangement faciltates the positioning of the eye to the eyepiece during normal use, and also enables observations to be taken up to 65 degrees elevation.
The Telescope has a field of view of 12 degrees and a magnification of four-and-a half diameters. The object glass is protected by rain shade, and the eyepiece is fitted with a rubber eyeguard.
A moderating glass is provided, which can be fitted over the eye lesn before taking observations on the sun, or other bright object.
Inside the telescope is a glass diaphram with horizontal and vertical crosswires against which are engraved and figured graticules from one degree to six degrees in each direction from the point of intersection of the crosswires. The first four degrees in each direction are sub-divided into 10 minutes graduations above the diaphragm, there is a coloured glass window through which graticules can be illuminated at night.
(i) Open sight- An open sight is mounted on the telescope bubble case.
(j) Plate, recordng deviation Mark.I- This plate os attached to the side of the elevation gear case and is for recording the difference between magnetic and grid bearings, the bubble coorection, place and date.