Letís take a Candlestick to Bits
The pillar style of telephone, better known as the candlestick, appeared in the 1890s. The design was smaller and neater than other contemporary designs, which usually had a large wooden box, usually fixed on a wall, containing the bell, magneto and induction coil.
The convenience of the candlestick was achieved by separating the transmitter, receiver and switchhook from the remainder of the parts which could be mounted out of the way in a wooden box, or Bell Set. Even though telephones with handsets became available about the same time, they remained bulky and heavy. The candlestick remained popular for many years because it could easily be carried about the room.
What we have here is a British Post Office Telephone No. 150, introduced in 1924 as a BPO standard design. Like many, this one was remanufactured from an earlier Telephone No. 2. Itís dated 1928. Before we take it apart, letís have a look at its construction and materials.
The telephone stands on a cast steel base plate. A good choice of material, adding weight to give stability. The base accounts for a good deal of the surprising weight of the whole telephone. Originally it would have been fitted with a hard rubber ring to protect surfaces from scratching. As rubber deteriorates with age I presume that any ring this one had has long since perished and fallen off.
The curved base and tubular shaft are two steel pressings. The shaft screws onto the base. The steel gives structural rigidity without raising the centre of gravity, preventing toppling. The black finish is stove enamelled. The switchhook fork is brass with a black oxidised finished. It is made from two pieces of strip joined at the body and spread at the far end. I canít see any benefit here of using brass instead of steel, except that it allows the decorative finish. The machining involved is minimal.
The shaft cap and transmitter support are a single brass piece, turned and machined to shape.
The superiority of brass at machining means that itís an obvious choice.
The transmitter is hinged using a separate brass piece not visible on this picture. The transmitter housing is black
Bakelite - the wonder material of the age - durable, heat and chemical resistant, with high electrical insulation.
The receiver cap is in brown plastic, probably a version of Ebonite, but the housing is brass with a brown coating, described as Ďenamelledí.
The transmitter housing which supports the internal chassis is now loose so you should not let it slide out of the stem yet. To allow removal of the internal chassis the dial cord must be disconnected.
Note the wire colours before you start. They donít always match those in the N250 diagram.
Push plenty of slack in the desk and receiver cords through the grommet in the base and slide the internal chassis and transmitter housing out.
Before we proceed further with dismantling the phone we need to disconnect the various cords after untying the lashing loop. Again make a note of the colours just in case. Typical GPO! The nuts are 3 BA (and the threads it appears).
I used a box spanner to loosen them. Washers tend to get lost over time and I thought there were a few short. 4BA washers from the spares box fitted fine.
The way the terminals are constructed is quite cunning. It avoids the need for a separate insulating support for the screws. Each hole is over-size and fitted with a fibre collar. The screws have fibre washers either side of the chassis making them insulated. Neat, eh? Note that the physical layout of the terminals matches the N diagram, even though the stamped letters donít.
Transmitter Support & Housing
This is a Transmitter No 22, which holds a Transmitter Inset No 13. Earlier models had a solid-back transmitter.
To remove the transmitter case from the hinge assembly, first remove the horn. This is the usual way of securing transmitter caps on telephones from the Bakelite era. Using a pointed instrument (here Iím using a blunt bradawl or even a well made drawing pin) press firmly into the hole at the base whilst twisting the cap anti-clockwise. Once free the three fixing screws are visible inside. Theyíre brass with a black oxidised finished, which is odd since theyíre entirely invisible and neednít be decorative at all. In fact they could well be steel. When you remove these you might want to note which way up the case goes. The three retaining lugs are not symmetrical and youíll want the hole in the lid to be at the bottom when you re-assemble it.
I havenít removed the cords from the transmitter housing as the ends are damaged and theyíll be difficult to get back under the screws.
The hinge assembly is oxidised brass. It will have to be split if you want to re-coat the metal. I was unable to release the screw. A more determined restorer might succeed!
If you remove the diaphragm it is important to slide it off sideways. This avoids distorting it. The tags on the new cord are slightly too big for the hole in the end of the receiver cover. They need to be bent a little to allow them through (and back again on re-assembly).
Donít polish the receiver cap too much. Itís black
Bakelite with a brown coating.
Too much polishing will let the black show through.
Furthermore, sources tell me that early models were fitted with a Dial No. 8. The N diagram quotes a Dial No. 10. This telephone has been fitted with the much more modern Dial No. 21 with its trigger action. A triumph for the Post Officeís design compatibility.
Iím not going to dismantle the dial further as dial maintenance is a separate subject,
so best covered elsewhere.
First the good points
Now the bad points
Summary & Commentary
Improvements in design of contact assemblies seems to be a lesson learned. Later designs, such as dial springs and relay contacts, used separate fixing screws from the screw holding them together.
The GPO candlestick Telephone No. 150 lived a somewhat charmed life. By the time it appeared GEC, Siemens and Ericsson were already developing self-contained Bakelite telephones with handsets. This culminated in the adoption by the Post Office in 1929 of the Telephone No. 162. Nonetheless the 150 remained in service for a surprising length of time. The Post Officeís policy of inter-changeability of parts means that this 1928 vintage telephone is still in full working order, despite having a new dial, transmitter insert and cords.
Pictures and text are all Copyright of Sam Hallas
Last revised: July 25, 2021