A Spotting Corrector was a component of a Dreyer Fire Control Table — a geared differential which permitted spotting corrections to the clock range to be tallied into a workable gun range. It also featured a means to add in a straddle correction brought over from the Dreyer Calculator. Eventually, its function was largely mimicked for deflection by the creation of the Deflection Totaliser.
The spotting corrector received the worm screw output of a range clock as an input, allowing it to know the Clock Range. A handle on the side allowed the person operating it to add an increment to the output range, and this was the means by which corrections were applied. The summed output was the Gun Range suitable to send to the director and guns for application on the sights. For convenient reference, a cyclometric Gun Range Counter displayed the present gun range digitally on the side of the Dreyer table.
A dial face marked to be 2000 yards around [Citation needed] with several hands on the top of the column-like housing was the means by which the corrections could be precisely entered. A pair of hands would spin as the spotting corrector handle was worked, and one of these hands could be quickly zeroed before applying the next correction. In this way, this hand became the one to watch when applying a correction, and the other showed the running tally of corrections applied to the input.
Let's take an example where the spotters aloft call down a correction of "Up 400". The man at the spotting corrector would zero out his correction hand and then work the corrector handle. Both the correction pointer and the total correction pointer would spin as he did this, and he'd use the recently zeroed pointer to see when he had worked both pointers through a delta of +400 yards. In this way, the total would be properly maintained, and the operator would be freed from any need to perform arithmetic. Moreover, the gun range counter would spin up as he did this, and the differential-based nature of the device meant that this correction would in no way interfere with any changes in gun range that occurred by action of the range clock.
I believe there was a third hand on the dial which helped the corrector separately maintain the straddle correction provided by the Dreyer calculator which was usually situated nearby on the bulkhead of the TS. I am not wholly sure how this worked, but I imagine that it was similar to the total hand, and usually did not revolve with the correction unless somehow engaged. In this manner, it would be engaged when the straddle correction was to be changed, and the old straddle correction denoted by its position would be altered by use of the corrector handle. After the straddle matched the newly dictated value, the straddle pointer would again be disconnected so its position would persistently signify the straddle connection in place.[Inference]
Pedalling Clutch Input
Another handle on the Dreyer table called the "Tuning Handle" provided a means to manually position the clock range pencil on the range plot or to "tune" it to the mean rangefinder range of the moment after the action had opened. It functioned by adding its value to the clock range worm screw through a differential. Since it altered the clock range worm screw, in order to keep the latter operation from causing the total gun range being tracked in the spotting corrector, a small lever near the tuning handle called the pedalling clutch would be worked, and this would cause another shaft which also entered the spotting corrector to simultaneously apply an opposite alteration in the gun range tally, again by means of a differential. In this way, the clock range could be altered however seemed right without affecting the output of the Dreyer table — in this case, the gun range.
Automatic Gun Range Output
In most Dreyer tables,[Citation needed] the gun range was transmitted from the spotting corrector by a flexible shaft where it arrived at the TS's bulkhead, terminating in the Range Master Transmitter. From here, by a Follow-the-Pointer operation, another man would cause the gun range to be transmitted to the guns and director(s).
Sending Gun Range to the Bearing Plot
On some Dreyer tables, a long shaft would also carry gun range over to the Bearing Plot where it might serve several functions. Some bearing plots[Citation needed] had Deflection Drums which had to be rotated to the proper gun range. On some tables late in their development,[Citation needed] a second digital gun range counter was remoted where it could be easily referenced by the man operating the Wind Dumaresq.
- Admiralty, Gunnery Branch (1918). Handbook of Captain F. C. Dreyer's Fire Control Tables, 1918. C.B. 1456. Copy No. 10 at Admiralty Library, Portsmouth, United Kingdom.