Zellweger is the brand name of an electric switching device used to control off-peak electrical loads such as water heaters. It is an example of carrier current signaling. Power stations transmit a ripple signal on the main transmission lines when off-peak rates start (often around 10 pm). This ripple noise is picked up by the Zellweger, which after a random delay turns the hot water heater on. The noise is often picked up by other equipment, especially audio amplifiers and stereos and the noise can cause problems with other electrical devices. Even some telephone lines can pick up the noise. The noise can be particularly obtrusive from some fluorescent light systems.
Newer electrical meters incorporate this technology into the meter. "Time of use" meters charge electricity to the current tariff within half an hour, giving customers incentive to run appliances such as dishwashers, pool pumps and clothes dryers during the night.
Power stations have plenty of unused capacity late at night but must keep running as they take days to shut down. Off-peak rates are used as an incentive for customers to use this surplus capacity and to reduce the amount of peak demand. This can produce cheaper power by delaying the need to build new power stations and reduce environmental impact. The random time delay in the Zellweger means that the power stations aren't hit with a huge demand when all the hot water systems turn on at the same time, rather the load is spread over a greater time period.
History in Australia
Originally time-clocks were used, however they can easily lose accurate time and are not easily adjusted for daylight saving time. Zellwegers were first introduced in Australia in 1953, however were not compliant with modern harmonic disturbance standards. The second generation was introduced in the 1970s and was more reliable.
A variety of devices can still be seen across Australia.
In at least some parts of Sydney, the ripple frequency is 1042 Hz. The signal usually consists of several bursts of a few seconds on and off, followed by a period of up to 50 seconds on. This is coded to affect only selected equipment. Occurrences are very frequent, sometimes several times an hour throughout the day, not just at evening and morning off-peak times. See example capture:
The bright line at 50 Hz is the power frequency, followed by odd harmonics at 150 Hz, 250 Hz and so on. The ripple being turned on and off is clearly visible at 1050 Hz.
Radioactive risk of one type of Zellweger meter
Zellweger ZE22/3 contain low-risk radioactive material, and must be only handled by authorised people as breakage of the glass tube could cause a dangerous situation by releasing the radioactive material. The device  reportedly contains a 'glow tube' containing tritium and radium.
New Zealand equipment
A few images are attached from an older electromechanical Zellweger ripple plant near Silverdale. Many of these Zellweger plants are still in use in New Zealand. Frequency used is 1050 Hertz, superimposed upon the 50 Hz mains. Solid state Zellweger equipment is used as well, which can directly inject into the 22kV or 33kV sub transmission mains instead of the 11kV mains as with these older Zellweger plants.