Is one permitted to change the level of an electric current on Shabbat in a device that was turned on beforehand? Is there any difference in principle between modifying the strength of a current and creating a new electric circuit?
During the Gulf War (in 1991), everybody in Israel was forced to sit at home waiting for instructions about possible missiles threats. This was repeated during the Second Lebanon War, in 2004. But listening to the radio can be a problem on Shabbat. Most prominent rabbis agreed that radios could be left on at a very low volume, and then made louder in case of a warning. The main thing was not to turn the radio on or off, but only to adjust the level of the electric current. (It should be noted that on a normal Shabbat, when there is no war, one should not listen to the radio at all, because it is forbidden to benefit from the broadcasting, which is linked to desecration of Shabbat in the studios.)
In the case of an electrical device that has what is halachically considered a "fire," such as the filament in incandescent bulbs or in heating elements, changing the level of the current is forbidden. In this case, adding or removing fuel from a burning flame is prohibited as part of the Shabbat labor of lighting or extinguishing a fire. Therefore, for example, one is not allowed on Shabbat to adjust the dimmer of a halogen lamp. However, changing the level of sound or the current in an electrical device where there is no "flame" does not involve any violation. Such prohibitions as "building" or "giving birth to" a new circuit do not exist in this case.
Based on this principle, one is permitted, for example, to adjust the volume of a two-way radio that is too noisy. There are also other applications which make use of the same concept.
It is important to emphasize that "modification" does not mean starting without any current at all, but rather a low setting at which a person can sense the results of the current. In a radio, for example, the volume should be at a level where a weak sound can still be heard. Similarly, for an electric motor (say in an electric wheelchair), the initial setting of the motor is one where its pull can be felt, and the controls either increase or decrease the power of the action. But there is never an "off" setting once the main motor has been turned on.
- An Electric Wheelchair – The chair is speeded up or slowed down by modifying the current, which is permitted on Shabbat.
- Hearing Aids – Speaking into a hearing aid only changes the level of current, and it is therefore permitted on Shabbat.
- Loudspeaker System – Speaking into the microphone only changes the level of current and is therefore permitted.
- Metal detectors – Detectors approved for use on Shabbat by The Zomet Institute have been designed such that the approach of a block of metal changes the level of current in the instrument. That is, sensing metal doesn't create a new circuit but rather changes the current flowing in an existing one.
- Security Cameras – Closed circuit video systems (which do not trigger other electrical activity) recognize any motion and translate it into a recorded picture by a technique that involves only a change in the level of current.
- Shabbat Computer Mouse – Moving the mouse moves the cursor on the computer screen because of a change of current level. Buttons on the mouse (for essential use only) are operated by mechanism of gramma (indirect action).
For more information (in Hebrew):
- Rabbi Yisrael Rozen, Modifying an Electric Current on Shabbat – a Halachic-Technical Principle and Some Applications, Techumin 26.
- Rabbi Prof. Zeev Lev, Creating a New Electric Current on Shabbat, Techumin 2.
- Rabbi Yair Meir, An Electric Wheelchair for Use on Shabbat, Techumin 8.
- Rabbi Yisrael Rozen, Closed Circuit Television on Shabbat, Techumin 14.
- Rabbi Yisrael Rozen, A Microphone and a Loudspeaker on Shabbat, Techumin 15.
- Rabbi Ben Tzion Chai Uziel, A Loudspeaker on Shabbat, Techumin 16.
- Rabbi Shlomo Zalman Auerbach, Radio Broadcasts on Shabbat, Techumin 16.
- Rabbi Yisrael Rozen, Closed Circuit Television on Shabbat, Techumin 25