After may years of development and vexatious discourse of technical and other forums, PLT manufacturers now admit that their technology is hardly state of the art but has its roots in an old an outdated and failed attempt at electrical central heating.
Not surprisingly such 'radio energy' devices had long been experimented with by early pioneers wishing to devise an efficient form of heating for those devotees of exotic plants with their large cast Iron conservatories. As early conservatories were not double glazed or has today's standard of glazing, a method had to be devise to protect collectors precious plants, many of them unique specimens, during the cold winters.
There were many options available including solid fuel, oil and paraffin - even gas was experimented with. These for the most part had their drawbacks mainly there were toxic fumes and sooty deposits which in an already polluted environment (mostly be cause of coal fires) caused great problems in both the health and the attractiveness of the precious plants.
In came electricity to the rescue: by this time more people were being introduced to the delights of electricity. However it was only the richer gentry that could really afford it and many of them had already their own generators attached to their houses. It is not known who was the first to have an electrical system of heating installed into their massive conservatory, but there is no doubt that a resourceful experimenter was suitably patronised and encouraged for what was developed was an absolute boon to exotic plant collectors and breeders of the country.
One particular experimenter and some say a particularly naive 'inventor',Mr Edward C deCorset, of Braintree in Essex is credited to have found inspiration while trying to develop and early type of multi-vibrator in order to obtain AC power source form DC which is what most installed generators of his day provided. He was no doubt aware of the better Loss factor of AC compared to DC. A fault occurred in one of the mechanical components in one of the designs he was testing. The result of this is that the rate of 'vibration' in the multi-vibrator became unstable and instead of the intended 50-100HZ there was a sharp rise and it is suspected that in the 'runaway' cycle the frequency of vibration became in the region of several KHz. The device did run for a brief time (about 2 minutes, though I think that is an exaggeration). What Mr de Corset did notice was that there was a distinct heating being induced into
the wiring on the output of the unit under test. There was not even a load on the unit that might have caused such a phenomenon.
Well further experiments ensued, and with the timely development and introduction of the latest thermionic devices (UK=valves, US=tubes) in what was seen as a pioneering circuit, Mr de Corset produced a unit with no moving parts which gave an output of electrical frequency energy that was both variable in power and rate with up to 5 selectable frequencies (100KHz, 160KHz 250KHz, 360KHz and 500KHz), which could be sequentially turned on to provide increased levels of 'invisible, smoke and smut free heating for the plant collector'. The supply to the Unit could be either AC or DC 'mains'.
The design of the unit is as follows: standard domestic wiring of the day was used, 6 closely coupled cables were attached at points to special ceramic plates about 4" square around the inside of the conservatory on the exterior wall. Each ceramic plate had a specially designed form of socket that was used in conjunction with additional Frequency Induced Self Heaters. The design of socket avoided accidental use by any other device of the day. The Heaters were normally only needed only in the coldest of weathers. They comprised of a ceramic plate about 24" by 12" with 18 mutually-inductively wound air-spaced coils of copper in a single layer. The coils were about 3" in diameter and 4" in height. A protective guard was placed over the unit. Each Unit could be plugged directly into the socket and locked in place. It is believed that there were different sizes of heater but only the specification for the 24"x12" one is known of.
The thinking at the time was that the heat induced in the wiring would not only conduct through the frame of the conservatory but owing to the ionisation effect of the resultant radiated energy, a 'skin' effect would occur with the glass making an 'effective' insulation layer. Most of the heat would remain within the conservatory. The temperature created was sufficient to maintain enough heat for the plants to survive which not affecting or cracking the fragile glass. The heaters would allow 'extra' heat to be delivered to particularly sensitive or valuable plants (in addition to their use in the coldest of weather). I shudder to think at the wattage actually used, that alone the harmonics induced.
All proceeded well until Mr de Corset was asked to install one of his units in a smaller wooden framed conservatory and then a local gentleman's home. It is not as you would think that there was a fire. No, it was the fact that what Mr de Corset had actually devised was in fact a primitive wide-band radio transmitter and a high-powered one at that. Where the cast iron conservatory had been successful in screening the signal Mr de Corset's units created and had not caused problems to the radio users, wood, brick and stone failed to screen the units and had let out all the radiated 'signal out and interfered with people's radios.
The Source was easily tracked down by the regulator of the day, the Post Office, and the Units shut down. As a result Mr de Corset had to let authority's know of all his installations and following further enquiries by the Post Office his and many other similar units were decommissioned.
In fact the owner of the house had remarked upon a most interesting phenomenon which occurred when he first used the central heating unit. He found that when he used his 'gramophone' the audio could not only be heard from the horn or speaker, in that the light bulbs would also seem to be ringing out with the audio and the wire filaments in them appear to vibrate 'mostly unusually'. Practical Wireless, a publication still in existence today, ran a couple of articles on it and put forward their own theories. In short they concluded that introducing RF energy into the mains wiring was bound to have the wiring act as an antenna, the audio from the record was leaking as a result of the fluctuations in the magnetic field of the speaker or horn which modulated the RF signal in the mains wiring of the wireless or gramophone, and that the junctions in the light bulbs were causing the signal on the mains to be 'rectified' and so the the audible effects noticed with both the filament and possibly at the same time the glass envelope could be vibrating, giving the audio tones the distinctive 'ringing' note.
It is believed that around 1930, some not so bright spark (presumably having read a back issue of PW) hit upon the idea that he could perhaps get away with a 'low' power version of Mr de Corset's unit to have music 'throughout his home'. Could this have been a forerunner of what is called a homeplug or powerline adapter. This chap apparently put together a basic unit with a primitive valve amplifier, either the output or the input would be plugged into a mains socket (please do not try this at home!) after 'isolating' it using a valve heater filament or speaker transformer (it is not clear from what remains of the sketchy 'circuit diagram') depending on whether he was 'sending' or 'receiving' his 'data' which would generally be in the form of audio from a wireless or a record player. He used a number of these devices around his house and in his shed/workshop.
Again, as a result of complaints to the Post Office about interference and 'strange voices and sounds coming through peoples' radios/loudspeakers, this illegal transmitter was traced and shut down> Mr de Corset was admonished about his 'experimental ' use of wireless telegraphy and had all units taken out of service.
A story many could learn from but . . . . . . .