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Met. Office Pilot Balloon Slide Rule MK 5

This is a Mark 5 Pilot Balloon Slide Rule issued in 1970.  The printed scales are engraved in the plastic faces. The slide rule has 4 captive cursors (Plexiglas windows and brass guides) that ride in slots on the top and bottom edges. Each can be moved independently of one another. Steel (other models had brass colored) knobs at both ends of the slide facilitate its movement.  This rule was used by the Met Office and has a Met Office Stamp and date.   

This slide rule does not contain a engraved notation directing the user to see a not on the back of the rule. however the note is engraved on the back of the rule as it is on the MK II, MK IV, and MK IV.A rules.  The previous Mark 5 rule reference to the note on the front of the rule, but lacks the actual note on the back of the rule. 

A large image showing the scales of the Mark 5 slide rule is also posted.  This rule is very similar to the Mark 5 rule and it is manufactured by the same manufacturer however there are some changes in the fiducial marks and the scales are about 1CM longer. 

Meteorological Office

Pilot Balloon Slide Rule, Mark 5


1. General Description

    The slide rule is used for computing upper winds from readings of the azimuth and elevation of a pilot balloon at regular intervals.  The theory of the method, with other details, is given in Met O 804 (Measurement of Upper Winds by means of Pilot Balloons, Fourth Edition, London, Her Majesty's Stationary Office, 1968), the quantities involved being as follows:-

    E  =  elevation
    A  =  azimuth
    K  =  graticule constant
    l   =  Length of tail
    m =  apparent length of tail in graticule units (See page 26 of Met O. 804)
    D  =  total distant traveled by balloon
    DE, DN  = easterly and northerly components of this distance
    VW to E, VS to N  = easterly and northerly components of distance traveled in the one minute interval
    h  =  height of balloon above starting point.

    The general relations between these quantities, as used in operating the slide rule are:-

    h = Kl sin 2E/2m = D tan E

    d = (Kl/m )(1/sec2E)

    DN = D cos A and DE = D sin A

    The Following scale sare incorporated:-

    A logarithmic scale on numbers, three time repeated, on both the upper and lower edges of the slider. 
    A logarithmic scale, twice repeated, on the upper part of the stock, identical with part of the slider scales.

    Sine and cosine scales, partly duplicated, on the upper part of the stock
    A secant2 scale, from 0deg to 63deg on the left part of the lower stock.
    A tangent scale , from 3deg to 84.3deg on the right part of the lower stock.
    A graticule scale (reciprocal logarithms), replacing part of the logarithmic scale on the lower left part of the slider. 

IMPORTANT.    The product of K, the number of graticule divisions per radian and L, the length of the tail in feet, is normally 1.2x105. In other cases adjust the length of the tail so that Kl/1.2x105 = 1

   Fiducial marks a b, c and d are engraved on the upper part of the stock, and marks E and ^ on the lower stock, for the conversion of units (see section 3). 

2.    Standard Procedure

    The standard procedure for the computation of winds by the use of pilot balloons and theodolite are described fully in Met O 804.  Three methods are described (a) winds by pilot balloon using a fixed rate of ascent (b) using a tail of known length and (c) using two theodolites.

3.    Change of Units

    The use of fiducial marks in reading values of wind speed directly in knots ar meters per second, etc., is fully described in Met O. 804.  When the slide rule is used for other purposes of in a non-standard way, care must be take that the fiducial marks still apply e.g. when using time intervals other than one minute or when performing a radar wind ascent where the ranges are in yards.  In these cases other reference points may nave to be found.  

    Inter-unit conversions may be made using the fiducial marks on the slide rule by directly reading between the marks thus:- 

    A -> B        converts    kts to ft/s
    B -> A            "            ft/s to kts
    A -> C            "            kts to m/s
    C -> A            "            m/s to kts
    E -> tan 45     "            ft to m or ft/s to m/s
    Tan 45 -> E    "            m to ft or m/s to ft/s

4.    Multiplication and division

    The additional logarithmic scale in the upper part of the stock permits the rule being used in a limited was as an ordinary slider-rule.

5.    Maintenance

    The rule should always be use with care and not handled with soiled or stained fingers.  If it is necessary to clean the surface warm soapy water should be used.  Abrasives, petrol or solvents must never be used.