Darke's high speed steam engine indicator no.421

Darke's high speed steam engine indicator no.421, fitted with patent detent and cord adjustor, spring and scale, in case with key

This arrangement of high-speed indicator was patented by Mr. E. T. Darke in 1879. To minimise the inertia of the indicator piston, its rod is made hollow while its motion is multiplied four times by a simple lever; the piston rod carries a sleeve, which slides on this lever, and the pencilholder slides similarly, but is guided in a vertical slot instead of by linkwork. The paper drum has an external clamping bar by which the paper is held; the paper is in a continuous roll within the drum, and enough for a single card is exposed externally and secured. The instrument is also fitted with Darke's detent (see inv. 1888-254).