Follow-the-Pointer (F.T.P.) systems were composite data receivers and settable inputs by which a remotely indicated value could be replicated and input into a local device to bring it into agreement with the demanded configuration by use of a lever or knob. In a manner of speaking, the man working an F.T.P. receiver acted as a human servo to apply the required setting no matter how weak its indication and how obstreperous the means of seeing it done. The Royal Navy employed F.T.P. widely in World War I, most commonly on gun sights where a remotely signaled range or deflection could be quickly matched on the sight.
If data can be transmitted and received reliably, the natural question becomes, "Why not simply then APPLY the data at the destination? Why have this sort of manual transcription step?". The reasons were several. One common issue preventing the application of the value was that the translation or rotation of slider or shaft to apply a value imposed friction and mechanical load upon the physical agent of communication, be it the position of the shaft of a motor, or a worm screw at the end of a flexible shaft. If this mechanical load were placed upon the motor or the system driving a shaft, backlash would occur which might reduce reliability, induce slippage, or simply overwhelm the muscle or rigidity of the systems upstream. Imagine, if you would, trying to power your car uphill by hooking up your pocket watch as a motor. How might this impact the accuracy of the timepiece?
F.T.P. also had to be considered alongside the methods it generally replaced. Some applications saw men standing alongside a data receiver, watching it for changes, and when they occurred, manually entering them into an entirely other control, noting its indication (which might differ arbitrarily from the visual model employed by the receiver). The result in such cases would be extremely fatigued men with their eyes nervously darting back and forth from receiver to equipment, struggling to ensure that both were in agreement despite their seeming intrinsic indifference to helping him succeed in his task. The propagation of the needed setting would be subject to error and delay. In other cases (such as sightsetting), the data conveyed enjoyed a more lush infrastructure such as a dedicated voice medium. Even here, however, the ease of meeting the simple cognitive charter of keeping a black pointer aligned with a red one moved by some remote agency trumps the verbal coaching alternative, particularly when changes become rapid or hard to anticipate.
In the Royal Navy in World War I, an F.T.P. gun sight received range and deflection data as trains of pulses sent over a wire by corresponding step-by-step range and deflection transmitters, with each pulse corresponding to a standard delta in the value being conveyed. The Royal Navy chose quanta of 25 yards for range and 1 knot for deflection. When the man at the corresponding transmitter dialed in an increment sufficient to advance his transmitter across the next quanta, a pulse would be sent and a servo motor at the F.T.P. receiver would cause an indicating hand (typically coloured red) to match the motion on the transmitter's dial face. The man at the F.T.P. receiver would then work a knob to advance a black hand reflecting the local setting so it would match that desired by the people at the transmission source (most often, the director or transmitting station).
Similar to the sightsetting application, the Royal Navy's elevation receivers and training receivers allowed the gun layers and turret director trainers, respectively to see the elevation or training angle the director was asking them to match. Their use of their hydraulic controls to lay the gun and train the turret would move their receiver's black hands as needed. When the red hands had the black hands positioned perfectly over them, the guns would be oriented correctly for firing.
Benefits and Drawbacks
F.T.P. receivers generally proved themselves in action and acted as the necessary sinews to tie the brains of a ship to its muscles. Certainly, their use of a human as an almost integral part of the system was part of their charm, as they threw manpower at the problem when a technological system of amplifying an indication to a more hearty impulse of effect did not yet exist and would not soon exist. The amount of training required to work them was scant, and while their precise nature differed from application to application, their fundamental nature did not and a man competent at working one was soon able at all. This reduced design and debug time necessary to create a reliably working ship of war.
F.T.P. receivers shared any frailties of their underlying data transmission technology (e.g., step-by-step systems might misstep, and this was both difficult to detect and tedious to remedy). Ultimately, their use was slowly eradicated between the wars in favor of synchronous and automated approached based on hunters or, in American service, RPC systems able to have director angles fed directly into the massive guns and turrets of the Iowa class battleships.