# Range Clock

A Range Clock is an instrument that can indicate a hypothetical range to a target and continuously adjust the presently indicated range by applying a range rate set upon it. It is not unlike a clock that can be set not only for the time of day, but also the rate at which time passes.

## Evolution from Clockwork to Electric

Although Frederic C. Dreyer alludes to a prior device designed by Fawcet Wray,[1] the first clock to be developed and employed was the Vickers Range Clock, which were windup devices with a clock-like face to indicate the present range. Knobs permitted the operator to

1. set the present range
2. set the range rate
3. wind up the spring if it ran low (a little telltale warned when this was needed)

With such a device offering only a visual display of the range, the model of use was for the operator to verbally or otherwise relay the present range onward to the guns, where others would keep their sights set according.

Later clocks such as those embedded within Fire Control Tables such as the Dreyer, Argo or Ford Rangekeeper needed to have more torque in the shafts expressing the range output than were needed to move hands around a small dial as in the Vickers. To meet this requirement, they usually augmented the clockwork power source with a human turning a following handle (following the pointer of the range clock, in a manner of torque relay), or by electric motors an alternate hand-crank in case of loss of electrical power).[2]

## Improvements in Variable Speed Drive Design

The variable speed motor that allowed range clocks to have a settable range rate usually fell under 2 basic designs. They were similar in that both employed constant-speed drive discs, and the adjustable speed came by choosing the radius at which their surface passed its motion on to an output shaft.

### Disc and Wheel

The Vickers clock and the Dreyer/Elphinstone clock used in Dreyer tables converted the constant speed of the drive discs by means of placing a roller wheel on the output shaft and permitted the distance between its point of contact and the center of the disc to be altered. The output shaft would spin at a rate proportional to the radius of the circles it would describe over the drive disc.

This design had a drawback, however; as the roller wheel was being repositioned on the drive disc to select a different range rate on the clock, it dragged "sideways" along the drive disc's radius. If you think of the roller wheel as a car being dragged sideways by a playful giant as it rolls upon a horseless merry-go-round, you can see that how it was prone to slipping when the rate was being adjusted. In light of this, range rate for these clocks was altered (either manually or automatically) in quickly-applied quanta, usually every time the desired range rate crossed a multiple of 25 yards per minute. This practice was called "discrete rate transfer".

### Disc and Balls

The variable speed drives within Argo and Ford equipment replaced the output shaft's roller wheel present in the earlier clocks with a system of roller balls that could not only spin along with the drive disc's rotation but also along the drive disc's radius as range rate was being adjusted. This approach allowed changes in range rate to be continuously applied and allowed clocks incorporating it to deliver a higher fidelity range output. Moreover, it also permitted simpler automatic mechanisms to supply them their range rate input.

### Performance Comparison

The requirement to use discrete rate transfer in Dreyer tables employing the Dreyer/Elphinstone range clock with its disc-and-wheel variable speed drive did not prove a significant impediment to accurate range-keeping during the war, as the errors introduced by discrete rate transfers in quanta of 25 yards per minute rather than continuous smooth rate transfers are inconsequential, provided that an attentive and suitably trained man is available to apply the rates as they change.[3]

The caveat to this would, of course, be if the disc-and-wheel variable speed drives somehow slipped under general conditions of use and not merely when being adjusted. There were two potential causes for this: oil finding its way onto the drive discs, or the unauthorized addition of custom appurtenances to the Dreyer table that would increase the load upon the range clock's output shaft. Both these threats were identified and addressed by explicit warnings to fire control personnel. Don't oil the range clock drive discs (use petrol), and think well upon adding anything that will impose a further load upon range clock output.[4]

The sufficiency of the disc-and-wheel drives did not mean that their weaknesses were tolerable as new systems were developed. The Admiralty Fire Control Table developed to supplant the Dreyer tables employed a variable speed drive with balls transferring rotation to the output shaft.