Manufactured by Thos Firth & John Brown Ltd of Sheffield, and operating like the well-known Brinell type, the Firth Hardometer used a ball of hard steel, pressed by a known force, into a test piece - the degree of hardness being estimated by measuring the resulting impression. The result was then compared to a chart, fastened to the front face of the head, and the reading noted. However, though simple in theory, due to the awkward nature of measuring accurately a small round depression - and the added difficulty of getting a sufficiently large one in harder materials - a pyramidal-shaped, diamond-tipped indenter was also supplied to improve the machine's versatility.
Hardness numbers were obtained by dividing the load in kilograms by the area of the impression in square mm, this calculation applying to both ball and diamond indenters - though in the latter case the hardness numbers were referred to as the Diamond Hardness Type. The load applied was controlled by a spring - the method being adopted to overcome the effect of inertia common to both dead-weight and lever principles - and the measurement of the depression made by a microscope, the size of which varied with the particular model of machine.
Limiting the size of sample able to be tested was the maximum gap between the face of the anvil and the head - on machines with an adjustable anvil integral with the base this was 8 inches and for those with a T-slotted flat-table anvil 10 inches  - and the 3.875 distance between the indenter and face of the supporting pillar. The minimum thickness of material able to be tested depended upon its hardness, the load and form of indenter used - the makers being rather cagey about this and offering to supply "fuller details upon request".
Of essentially simple construction and reasonable cost, the Firth Hardometer consisted of a base in cast iron machined with a hole into which fitted a round steel column able to be rotated about it and locked. The column was fitted with rack and pinion gearing by which means the head - the front part containing the indent mechanism and the rear a measuring microscope - was raised and lowered. The indent was made with the head positioned at the front and the assembly then rotated to being the measuring microscope into position. The standard base table (the anvil) upon which samples were held, consisted of a flat surface (plain or tilting) carried on a column able to be rotated in a hole bored in the front of base casting. As an alternative, the base casting could be provided as a much larger unit into which were machined two T-slots, these being intended to hold vices or other accessories - the makers offering as an extra a hardened and ground a V-block combined with a flat table
Enclosed and protected, the patented indenting mechanism (Nos. GB 280292 and 464005) applied its load through a calibrated spring and trip mechanism, the makers claiming that by this means only an exact, measurable force could be applied. One drawback of the ball method on harder materials was that, above 550 Brinell, the balls tended to deform and the impression made became inconsistent. In addition, if high-quality balls from the maker were not used, the results could be useless and hence it was recommended that the testing point - be it ball or diamond - be substantially harder than the material tested.
When using the diamond indenter the makers recommend the 30 kg. capacity model and for use with steel balls the 120 kg. version - though either would take both types. For example, if used for testing materials up to 550 Brinell using ball, and only occasionally for use with the diamond on materials over 550 Brinell, the 120 Kg. version would have been suitable, but on materials usually over 550 Brinell (using the diamond indenter) and only occasionally for steel-ball work, the lighter version would have sufficed. However, any sensibly run, well-equipped toolroom would have purchased both types.
Models available in the late and 1920s and into the late 1930s consisted of the C.2, with a separate microscope, and the identical D.2 but with an integral unit. Both C.2 and D.2 could be supplied with a 120 Kg. load cylinder and 1 in. microscope for testing medium and hard steels down to 1 mm thick and soft metals down to 2 mm. Alternatively, with a 30 Kg. load cylinder and 1/2" microscope, they could be used to test medium and hard steels down to about 0.5 mm thick or soft metals down to about 1 mm. With the addition of a diamond indenter it was claimed that either the C.2 or D.2 would be able to test the hardest of hardened steels. With a growing demand from the late 1920s onwards for a tester able to handle very thin specimens, especially those surface-hardened by the Nitrogen process, the makers produced the E.2, a modified 10 kg version with a 1/4" microscope and standard-fit diamond indenter. The E.2 was supplied, at first, in several types, to special order, its intended use being with a diamond indenter on both very thin samples and those steel parts with a relatively thin carburised layer of not less than 0.009 to 0.0095 inches thick - or on nitrided surfaces not less than 0.008 to 0.11 inches thick. In its ordinary form the E.2 was equipped with the universally adjustable flat-topped table, but the T-slotted base could also be fitted.
By the early 1930s Firth testers were available with a small microscope projector head, this fitting allowing the operator to work seated and use both eyes to read a scale projected onto a ground glass screen - its brilliance varied by a rheostat. As a result, fatigue was reduced and accuracy of readings improved.
By the late 1930s the model range had become the Type F1, a fixed-load version fitted with either a 120 or 30 kg cylinder; the F2, also of fixed load with a 10 kg cylinder and a new model, the G1, a versatile variable-load machine - able to work on a wide variety of materials and hardening depths - that could be adjusted to work between 2 and 40 kg and equipped with a large dial to show the load set. The range of applications was as before: the 120 kg version able to be employed on all but thin sections and the 30 kg model for thin sections and steels that had been surface hardened by either case-carburising or nitriding - providing the hardening was a minimum of 0.016" thick. It was recommended that the 30 kg model be reserved for testing steels above 550 Brinell using just the diamond indenter, the life of the diamond likely to have been relatively short if used under the very much heavier load of 120 kg. The 10 kg Type F2, with its diamond indenter and ability to test very thin materials (or thin layers of hardening on normal components), replaced the original E.2..

A Firth Type D.2. As advertised during the early 1930s fitted with a 120 kg. load cylinder, adjustable flat or tilting anvil and a microscope mounted on a floating bracket. Also included with each new machine were three dozen balls, a ball-holding device, a standard hardness check piece, the necessary hardness tables "B" and "C " and a waterproof cover. A diamond indenter was  charged extra.

An earlt 1930s Firth Type D.2. fitted with a 30 kg. load cylinder, a diamond indenter, the heavier T-slotted anvil and a microscope mounted on a floating bracket; also shown is the maker's hardened and ground V-block. Included with each new machine were three dozen balls, a ball-holding device, a standard hardness check piece, the necessary hardness tables "B" and "C " and a waterproof cover. The diamond indenter was not standard on this model but charged extra.

An early 1930s Firth Type E.2. fitted with a microscope projector head and the brightness-varying rheostat, a 10 kg. load cylinder, a tilting or flat table, a standard-fit diamond indenter, a standard hardness check piece, a special hardness table, a microscope mounted on a floating bracket and a waterproof cover

Firth  Type G.1 from the late 1930s to 1940s. This was a variable-load model with a range from  2 to 40 kg. A 12-volt supply illuminated the dial

Probably taken in Sheffield at the works of the manufacturers, Thos Firth & John Brown Ltd., from the hairstyle the picture dates from the late 1930s

The diamond-tipped impact
assembly