of 10 (strictly, in alternate scales of 2 and 5 ) and is purely electronic:
auxiliary circuits which can operate more slowly however, use also mechanical
relays and uniselector switches.
The earlier Robinsons counted in four electronic scales of 2, followed
by four mechanical relay scales of 5.
Colossus I counted electronically, in three scales of 2 followed by
four scales of 5.
Copying machines, whose speed per letter is much less, generally
employ mechanical relays, but Miles A is largely electronic and Tunny and the
decoding machines use a few valves.
(f) Use of standard components.
Many features recognised as desirable in Tunny-breaking machinery were
not incorporated because they require equipment which was either non-standard or
not readily obtainable, e.g. six-impulse tape. Indeed it is a recognised
principle that a machine which can be assembled from standard parts, even though
more complex, is preferable to a machine requiring special parts. This is due in
part to availability, in part to the probability that the special parts will not
work properly. This is one advantage of electronic equipment: the amazingly
reliable counters of Colossus are of novel design but do not need special parts,
being made from standard valves and other standard equipment.
(g) Note on the source of machines
All machines were provided by the Post Office Engineers except the counters
of Heath Robinson, and some copying machines due to TRE. The maintenance of the
TRE machines by P.O. Engineers was never officially authorised, a most
unsatisfactory state of affairs, in consequence of which, despite their relatively
simple character, they are less reliable than Colossus.
(h) Readers and Reperforators.
There is one example of technical vagueness in this account of which warning
must be given. The five impulses which constitute a teleprinter letter are
transmitted over distances successively, not simultaneously, for otherwise five
separate wires or other carriers would be required. Within a terminal office,
however, there is no objection to the use of five wires; in some tape readers and
reperforators the five impulses appear simultaneously, in others successively.
Both types are used for Tunny cryptography, though for this purpose successive
impulse apparatus has no advantage except availability: it is clearly much easier
to add and permute simultaneous impulses.
The hand perforator, the Insert machine, Junior, Garbo, and the punch of
Colossus 6 use simultaneous impulses.
Angel, Tunny, and the decoding machine use successive impulses.
Miles (including Miles A) reads the five impulses simultaneously but sends
them successively to the reperforator.
Readers which produce five successive impulses are supposed to be called
Reperforators which receive five impulses simultaneously are supposed to be