Whilst the most common War Department marked tools that turn up are spanners, for Air Ministry marked tools, measuring devices seem to be some of the easier items to find. A few weeks back I was lucky enough to find a pair of Air Ministry marked callipers in a tool box and for the princely sum of £2 they were mine:Callipers are mainly used for measuring the diameter of cylinders, although have other uses in light engineering work. This pair have a crown and AM marking on one of the legs, together with a maker’s name of Buck and Hickman Ltd:Interestingly they also bear a /|\ mark and an inspector’s code 60 on the reverse:Using callipers correctly requires a little practice and a 1930s engineering manual gives this advice:
Measuring with Callipers
Callipers, either spring or firm joint types, are convenient tools for measuring a number of jobs which are not required to be extremely accurate. Firm joint callipers have two legs fastened together with a rivet or bolt of a special design. To give a smooth joint thin fibre-washers are interposed between the legs, and when purchasing callipers this point should be looked for.
The legs of inside callipers are curved outward at the extremities to facilitate measuring small holes, whilst outside calliper legs have a large curve inward to increase their capacity for large work.
Firm joint callipers are adjusted approximately by the hands and then set to the “feel” of the work by tapping them on a metal surface. It is common practice when opening callipers by this means to tap the top of the joint. It is better, if possible, to tap the inside of the legs, as repeated blows, though light, tend to burr the edges of the joint. Care should be taken when using callipers to hold them square across the job, or an incorrect reading will be obtained. The interference between the work and the instrument should be very slight as distortion of the legs occurs if force is used. The application of callipers for good results calls for a certain amount of practice.
The book also illustrates another use for this instrument: