N - Diagrams
Circuits and diagrams of older telephones and telegraphs can be found in text books, such as Herbert's Telegraphy. Herbert and Procter's Telephony and Atkinson's Telephony are all generously provided with diagrams of the most common instruments, whilst older books on the subject (and general electrical works too) frequently have circuit diagrams. For telephones made outside the UK, books are the first place to look as well. Ralph Meyer's book 'Old Time Telephones' covers all commonly used American telephones and many of these were exported to Europe and the rest of the world (or made under licence there). In other countries there are standard textbooks on telecommunications in which you will find circuit diagrams, and these books will be found in libraries there.  

All British Post Office telephone and telegraph instruments were wired to standard circuits, which were printed in reference books. Up to the first decade of this century these were in hardback pocket-books - first a combined telegraph and telephone book, then separate issues for telephones and telegraphs (red for telephones, black for telegraphs; up to 1912 the separate National Telephone Company also produced a similar book). As the number of diagrams proliferated, they were produced in loose leaf-form, the most common being the 'N series' of telephone circuits (universally known as N Diagrams).

A large selection of N,Q and miscellaneous diagrams can be copied for members, see the THG Journal, or e-Group. N-Diagrams may be sometimes be obtained at swapmeets. Technical libraries, such as BT Archives in London is another useful resource, and well worth a visit.

Alternatively, many of the more common N diagrams may be downloaded in PDF form from Sam Hallas' website, click here to see the list.