Most early block instruments were of the "2-position" type, ie they could show either of two indications only. This arrangement originated from the early days when block sections were normally "open", ie a signalman was free to send a train forward to the next signal-box at any time unless the section was already occupied. The instrument therefore needed to show only whether the block section was clear or occupied, and this was usually done by some form of needle or arm indicator.

Later it became the practice for block sections to be regarded as normally "closed", so a signalman could send a train forward only after receiving a specific acceptance from the next signalbox. This meant that block instruments now had to give 3 indications, namely "Normal" (often called "Line Blocked"), "Line Clear" or "Train On Line". In most cases this was achieved simply by making the 'normal' position of a 2-position block instrument mean both "Line Blocked" and "Train On Line" - an expedient that required the signalman to be extra careful and meticulous in his train register entries.

Eventually various designs of 3-position instrument emerged, where the indicator could be moved to any one of 3 different positions corresponding to the 3 section conditions. Although this type of instrument was used by the Southern for most of their new work, including a programme of replacement of much of the older inherited equipment, 2-position instruments were remarkably long-lived in some areas until fairly recently, when the final sets disappeared from the Uckfield line.

[NOTE: Just to confuse matters, in SR and BR(SR) days the terms 'open' and 'closed' block were used to identify whether or not the signal governing entrance into the block section could be cleared without obtaining 'Line Clear' from the box in advance.]

Block instruments are also described as being of the '1-wire' or '3-wire' pattern, depending upon the number of line wires required to connect the instrument in one signal-box to its counterpart in the next box. An earth 'common return' was used originally and this was not included in the count of the number of wires, even when it became the practice to provide a specific 'earth' return wire.

In a 3-wire instrument one wire would be for the 'up line' indicators, the second for the 'down line' indicators and the third would be for the block bell (usually separate from the actual block instrument). The 'normal' condition would be for there to be no current flowing in any line wire and the indicators would fall to the 'Line Blocked' position under the influence of gravity. Displaying 'Line Clear' or 'Train On Line' would a require continuous flow of current of the correct polarity. Operation of the block bell would not affect the block instrument.

A 1-wire instrument had only one line wire, plus the earth return. This wire worked both the block bell (usually built into the block instrument) and the indicators for both directions of travel. The indicators were held in position by magnetic fields and their position would be changed by reversing the polarity of the intermittent current on the line wire, which flowed only when the bell plunger (or some other switch on the instrument) was pushed. Operation of the block bell therefore was a critical part of the functionality of 1-wire block instruments.

Preece's Instruments
William Henry Preece was Telegraph Superintendent of the L&SWR from 1860 until about 1870, so not surprisingly the L&SWR was an extensive user of the block instrument which he designed. These were 2-position instruments, supplied in both 1-wire and 3-wire forms, which could be distinguished visually by the shape of the main indicator aperture - round for 1-wire and rectangular for 3-wire instruments. The block bell was mounted on the top of the instrument behind the semi-circular front. Preece instruments were remarkably long-lived - the first was installed at Exeter in 1862 and the last examples were not taken out of use until about 1970.

Preece designed the indicator of his instrument to look like a miniature semaphore signal, and this showed the status of the line to the next box. Separate from the instrument itself was a switch in the form of large lever-like handle, which worked the corresponding indicator in the next signal-box - the position of this switch handle was the signalman's only indication about trains coming towards him. At one time the semi-circular panel at the top of the instrument contained a 'disc' indicator, but these were all taken out of use long ago.

Right: A 1-wire instrument in the ex-S&DJR TemplecombeNº2 Junction box in 1957.

photograph by Dr Ian Scrimgeour, courtesy Signalling Record Society

The case for the 'trains coming' indicator also had a front-mounted drop-handle, which was moved to one side or the other in order to work both the indicator above it and the corresponding one in the next signal-box. This handle was held in either position by a peg put through its shaft - hence the name of 'pegging' instrument. Conversely the indicator-only instrument, which was connected to the pegging instrument at the other end of the section, was known as the 'non-pegging'. Just to confuse the issue, some early 'non-pegging' instruments actually had pegging handles as well to enable them to double-up as 'speaking telegraph' instruments!

Rather like the S&DJR the SR instrument also had its origins in the old 'speaking telegraph', to the extent that the SR apparently manufactured their early instruments by re-using spare or redundant telegraph instrument cases! (There is some suggestion that they also used old Sykes Lock-and-Block instruments.) On the front of the instrument was a round knob, rather than a drop-handle, which worked the commutator controlling the line wire circuits. Above this was a circular indicator, worked by the commutator, which related to trains coming towards the signal-box. A second circular indicator, for trains going away from the box, was mounted on a stalk above the main instrument case, and this could be swivelled around for ease of viewing by the signalman.

On the front of the instrument to the left of the knob there was a push-button which, via various proving circuits, operated an electro-magnetic lock inside the instrument that prevented the commutator being turned from 'Normal' to 'Line Clear' unless it was safe to do so. (Some drawings suggest that in the original design the push-button in fact was a tapper key built into the bottom of the instrument.) In later years this push-button and the electro-magnetic lock appear to have been omitted or removed. An addition to the original design in 1935 was the provision of a manual "Line Occupied" slide to the right of the knob, which the signalman could use to lock the commutator in the "Train on Line" position, eg as a reminder when a train had to be held outside the home signal.

photograph by Mark Westcott