Workshop Hints
The Hipp Toggle (and the Post office clock No. 36)
The British Horological Institute has archived and edited the following from e-mails sent to the Clock/Clockers mailing lists on the Internet. The information here does not necessarily indicate a method approved by the BHI, we are only publishing this digest so that others can decide for themselves whether the methods listed below will suit them.
From: Ken Benstead, Alan Heldman, Eliot Isaacs FBHI, Steve Thackery, Kevin Eva
The Hipp toggle is used on mechanical/electric master clocks. The aim of this device is to provide an impulse to the pendulum when it's amplitude falls below a set minimum. This device allows the pendulum to swing without any interference most of the time and get an impulse only when it needs it.The best use to which it was applied is in the master clocks which until quite recently were installed in every telephone exchange in the UK. Known as the "Clock Post Office Number 36", these had a 1 second invar pendulum. These clocks were unbelievably reliable, as indeed they had to be.
Mattheus Hipp, a Swiss, invented this in 1838, and clocks to this principle continued to be made throughout the 19th Century.
The pendulum has a little piece of hinged piece of steel mounted about 1/3rd way down the pendulum from the suspension, this is the Hipp toggle(3). The pendulum will also have an iron block (Armature) fixed usually about 1/4 way down the pendulum(2). This block lines up with an electromagnet(1), but it never will touch the electromagnet.
Below the pendulum and to one side of the centre of it's swing, is a normally open 'microswitch' (5 & 6).The classic Hipp 'microswitch' is two flat leaf springs,(like suspension spring material) say 1/2" by three inches, horizontal, separated by an insulator block at one end. At the opposite end of the upper leaf spring is a block of agate (4), with a vee cut into it with the points of the vee pointing up . The inverted V has little notch cut into the apex.(This makes it like an inverted W). This ensures that the trigger either lodges in the notch, or runs down the slopes on either side. The slopes are in fact quite gradual. The microswitch is wired in series with a battery and an electromagnet. 
The electrical contacts are tungsten or platinum, and are pushed together when the Vee is depressed. There's enough additional flex in the whole assembly that it can cope with the rare occasion when the vane catches on the upper edge of the Vee. More importantly, the "geometry" of the system is such that in the normal contacting process, the two electrical contacts don't merely touch, but because of the bending of the leaf springs, they WIPE across each other a little bit, thus dealing elegantly with the problem of corrosion building up from sparking from back EMF when the momentary current in the magnet stops. To overcome any possibility of sparking while the switch is activated, the clock 36 has resistor-capacitor spark quench circuits across all it's contacts.
When the pendulum has a healthy swing, the Hipp Toggle will rake over the VEE but will continue moving and will completely clear the VEE; and on the return path, it will do so again, etc. As the amplitude of the pendulum finally dies down enough, the Hipp toggle only drags part way across the VEE block when the arc ends and reverses; and as the arc reverses, the little metal toggle will get caught in the notch of the VEE and will push the VEE block down, closing the microswitch, energising the electromagnet, which pulls the piece of iron on the pendulum rod, and thus impulsing the pendulum.
This cycle repeats, with the toggle catching in the VEE and causing an impulse only when it is needed, from 15 to 30 swings depending on each individual clock. Unlike most other master clocks, (Synchronome, Pul-Syn-Etic, etc.) the clock is almost completely silent in operation..
There were a number of contact sets and ratchet wheels fitted as standard to the Clock 36. On the right side (as you look at it) is a 30 second wheel (with 30 teeth, two of them diametrically opposite each other cut deeper so the pawl drops a little deeper and catches the electrical contacts (remember, it takes two seconds to complete a swing!). On the left is a 6 second wheel. At the top are two springs operated directly by the pendulum, which are connected together so as to generate a one second pulse. So, altogether a Clock 36 produces 1 second, 6 second and 30 second pulses. All these pulses were sent to a relay set (normally mounted above it) which provided the high drive current required by the exchange.
The 30 second pulses drove the stepper clocks all round the exchange building. All three pulses were fed into the innards of the exchange to work various magic associated with local call metering, etc.
Because the electrical impulse to the magnet to impulse the pendulum is "as needed" it will not be at precisely regular intervals, and cannot be used to signal secondary dials.
Typically the dial at the master Hipp clock has a ratchet wheel which is propelled one notch at each swing of the pendulum, by a finger on the pendulum, and this connects on to a gear train to display seconds, minutes, and hours.
Sometimes, the pendulum might swing just far enough to drag the toggle to the very edge of the "V" block and then starts its return swing, it does happen (but not often)! The contacts are lifted higher than normal but the flex in the contact springs copes with it easily. What almost invariably happens is that the toggle falls off the edge (either into the bottom of the vee, or down the outside edge of it) part way through the stroke, with a delightful boinngg! sound.
There's a nice little book, "Electric Clocks & How to Make Them", edited by Frank Hope-Jones, which describes two models of Hipp clocks, one with a prolonged impulse, which the designer claimed gave a smoother action to the pendulum. Reprints of this book are still available from most of the UK booksellers.
