machine position Y will encode as X, and this law is known as the "Reciprocal" property of the machine.
6. Another important point is that a letter cannot encode as itself (i.e. A cannot encode as A). If this were possible it would imply that a point on the reflector was connected to itself which is not so. This property of the machine - known as "Non-crashing" - is very valuable in helping to place cribs correctly. [See chap. ].
The Wheels and the Turnover System.
7. On the right hand side of each wheel (as it stands in the machine) is a toothed cogwheel; on the left hand side is a metal TYRE or RING marked with the numbers 1 to 26 (or letters A to Z) in order, settable in any position by means of a CLIP and with a TURNOVER NOTCH (or 2 notches in some cases) on the extreme left hand edge. There are 8 wheels, all differently wired, and with the notches differently placed on them. The motion of the machine is controlled by the three TURNOVER PAWLS at the back on the machine which engage the cogwheels and turnover notches. The effect is as follows. The right hand wheel turns over once each position irrespective of the motion of the other wheels. The middle wheel turns over whenever the right hand wheel is in a turnover position. The left hand wheel and the middle wheel turn over whenever the middle wheel is in a turnover position (a wheel is said to be in a "turnover position" when a notch on the tyre is ready to engage its appropriate pawl).
8. The "window Readings" (i.e. the wheel position as read off through a window on the cover of the machine) of the turnover positions on the various wheels are I Q, II E, III V, IV J, V Z, VI, VII and VIII each M and Z. Suppose for example that the WHEEL ORDER is 8 6 2 (i.e. wheel 8 is the left hand wheel, 6 the middle wheel, 2 the right hand wheel) then a series of successive positions would be
ZKC, ZKD, ZKE, ZLF, ZLG .... ZLY, ZLZ, ZLA, ZLB, ZLC, ZLD,
ZLE, ZMF, ANG, ANH etc.
9. The idea of the turnover system is clear; it is to ensure that the machine does not return to its starting position until it has gone right round through all (or almost all) of its 17,000 positions. Three points are especially worth noticing as of great practical importance.