Taken from the book "Electricity and the Sabbath" by Jerimiya Ibn Asher and Rabbi David Shure (http://www.science-halacha.com)
We must first explain the halachic concept of the work of Building and Demolishing. The Biblical prohibition is to build a permanent structure or create a permanent implement. If one put together all the parts that are needed to make the implement and this implement is designed to last six months or longer, it is considered a permanent implement. This action is forbidden by Biblical law. If it is a collapsible implement, and, in normal usage it does not have to remain this way for any length of time, then it is permitted to put it together and dismantle it. If, however, it is made to stay up for a short period of time under normal usage, then it is Rabbinically forbidden.
An implement is made to be used. Only permanent parts of an implement have the halachic status of being parts of the implement. Assembling the last permanent part that enables the implement to function is considered completing the implement. Usage of the implement sometimes includes making a temporary change in the implement in order to use it. For example, a candle is completed as an implement when the wick is prepared to burn. Burning of the wick is considered using the candle and not making the candle, even though the candle is useless if it does not burn, and the burning is an internal change in the candle itself. The same would be true of a light bulb. Since it is the common practice to turn it on and off, the implement is considered completed as a light bulb when it is ready to be turned on. However, in the case of long- lasting bulbs, which are specifically manufactured to be placed in the entrance of an office building to remain on until the fluorescent burns out, the lighting of the bulb is permanent and considered finishing the implement.
The same reasoning holds true for watches. A mechanical watch is considered a finished implement when it is ready to be wound. The winding itself is temporary and is in the category of usage of the instrument. A transistor watch, when turned on, will work more than six months. Therefore, the watch's movement is permanent and is part of the implement. Consequently, one does not violate the Biblical prohibition of making an implement on the Sabbath unless he turns it on. But he does violate a Rabbinical prohibition when screwing in some parts of the watch. This is considered doing part of a work which in this instance is making part of an instrument.
By custom, we forbid the winding of a watch on Sabbath. This is because it appears as if one made a temporary improvement in the watch that will last for a short time. Consequently, any implement that, when turned on, will last for a short time, is forbidden to be turned on. If it can be turned on or off at will, like a radio, and it does not have to remain on for a short period of time in normal usage, it is forbidden to turn it on because he is producing sounds with an instrument, but not because he is making or improving an implement. An umbrella is forbidden to use because he is making a tent (a derivative of "building") being that in its normal use to go from place to place in the rain, the umbrella must remain open for that period of time.
If ones action can be interpreted in two ways, either that he is making an implement or that he is not, for example, if he breaks a splinter off a piece of wood, the piece may be useless or he may need it as a toothpick which is an implement. If he did not intend to use it as a toothpick, then it is not an implement and this action is not forbidden.
An automatic self-winding watch that has stopped and is afterwards worn before the start of Sabbath will not continue on its own until the end of the Sabbath. Therefore, one's hand movements cause it to continue to go on the Sabbath and it is the same prohibition as winding a watch on Sabbath. If however, it would continue with its own stored energy to function for the entire Sabbath, then he does not intend to wind it when wearing it and it is permitted. The same is true of a wind-up watch that is still working but will stop on Sabbath, if he winds it so that it will not stop, it is forbidden. If his intention was that it should also continue working after Sabbath, then he is violating the Rabbinical prohibition of preparing something on Sabbath to be used after Sabbath.
The "Seiko Kinetic" wrist watch creates, through the movement of the hand, a weak electric current that powers a quartz movement, and also stores this energy for future use when the watch is at rest. The weak current disqualifies it from the work category of fire. This type of watch is to be compared to an automatic mechanical watch. The electrical energy being created through movement in the "Seiko Kinetic" is to be compared to the mechanical energy being created through movement with the automatic wrist watch. The electrical energy being stored is to be compared to the mechanical energy being stored in the automatic wrist watch. Therefore, the halacha is the same with both watches.
However, where there is no discernible change in the mechanism itself, for example, opening the refrigerator door for a period of time will cause the motor to work longer. There is no discernible change in the motor mechanism and consequently, no improvement in the implement. Functionally, however, there is an improvement in the refrigerator. Letting it go longer cools the refrigerator properly. Because of this action he lets in warm air thus making a change in the situation of the refrigerator which is corrected by the motor running longer. Therefore, he is guilty of making an improvement in the refrigerator but not in the motor. It is also forbidden because he is causing the motors to generate more sparks, since it is working longer. In Chapter VI we will discuss the circumstances and methods how a refrigerator may be opened on the Sabbath and the reasoning behind those who permit it.
A solar-powered watch is run by the current produced by a battery that is recharged by current produced through the exposure of the watch to light. If there is enough electrical energy stored in the battery to keep the watch moving all Sabbath then he is permitted to move it from a dark room to a lighted room. If there is not enough energy to keep it going all Sabbath, he may not move it from a dark room into a lighted room but he is permitted for the rest of Sabbath to remain with it in a lighted room since he is not doing an action to improve the functioning of the watch.
Making an implement that will last for only a short time is not forbidden on Yom Tov. Therefore, it is permitted to wind a watch on Yom- Tov even if it is not working, providing that he needs it for Yom- Tov and has no other watch to use.
If he paints or glues the hour numbers on the face of a clock he violates the work of improving an implement and the work of writing. If he places the clock movement into a casing he violates the work of improving an implement. If he corrects the hands of the clock while it is going he violates the Rabbinical prohibition of improving an implement.
The rules of direct and indirect action pertaining to the works of Sabbath are described in Responsa Achiezer, volume three, Chapter sixty. A direct action is done by his hand or by an implement deriving its power directly from his body. An example of a first degree indirect action would be pressing a button that starts a motor to perform a work. Although there may be several indirect actions, one by first allowing the electricity to flow into the motor and then the motor to drive the gear system of for example a sewing machine that can sew automatically. Nevertheless, since it was built as one unit for a specific job or jobs, all indirect actions done to achieve this purpose are considered as one indirect action of the first degree. The rules of indirect actions relating to the work categories of the Sabbath are not like those relating to other prohibitions. If an indirect action of the first degree is done in the normal way and by this action the resultant reaction is definite, then he is as guilty as if had done a direct action. For example, if he put a plate of water under the sparking candle to quench the sparks that fall into it. If the sparking candle will definitely spark and consequently, the plate will definitely quench the sparks, then he is guilty, since this is a method that is used to quench these type of sparks. If however, he puts the same plate under a candle to quench the candle when the burning wick reaches the water, it is considered an indirect action of the first degree that is forbidden Rabbinically but not Biblically. The difference is that although it will definitely quench the candle but the normal way to quench a candle is by quenching the burning wick directly and not by having the wick burn until it reaches the water. This Rabbinical prohibition of an indirect action of the first degree is not to be confused with the Rabbinical prohibition of doing a Biblical work in an unusual manner ("shinuy"). This case cannot be classified as a "shinuy" because it is not that much of a deviation from the norm. The reason it is an indirect action is because the fire continues burning on its own energy by consuming fuel in the direction of the water.
From the previous discussion we find that an indirect action of the first degree in not considered a direct action, although he is as guilty as if he did the work by direct action. If however, the indirect action is not done in the usual way, the torah did not forbid it and it becomes an indirect action that is only forbidden Rabbinically. However, for a direct action not to be forbidden Biblically the action must be done ia a very unusual way ("shinuy").
An indirect action of the second degree is not forbidden by the Torah even when it was done in the usual way and the reaction is definite. This is discussed in Tractate Sabbath, page 120: If he makes a barrier of water filled barrels before a fire so that when the fire reaches the barrels they fracture and release their water contents to quench the fire, he has done an indirect action of the second degree that is not forbidden by the Torah. The first indirect action is that the fire approaches the water using its own energy burning its way along. The second indirect action is the combination of fracturing the barrels and releasing the water, consequently, quenching the fire. Although the fracturing of the barrels is a direct result of the fire yet it is considered a different action because there are other ways to fracture a barrel that are not related to heat. Therefore, fracturing the barrel is related to the water flowing onto the fire and quenching it thereby. Tosofos there brings an opinion that if he makes a wall of ice or snow before the fire it is also a second indirect action. The melting of the ice is comparable to the breaking of the water barrels. The other opinion disagrees and writes that melting is part of the action of the burning fire because this melting can only be done through heat. Therefore, what is the difference between the action of the fire burning the wood or its continued action of melting the ice. There is only the first indirect action of the fire burning by itself to be quenched by the water when it reaches the ice. The first opinion holds that since kindling a fire and cooking the ice are considered different work categories of the Sabbath therefore, they are different actions also. Another answer is that the water falling on the fire is not directly due to the fire, therefore, it is considered a second indirect action.
The fact that there is a time interval between the action and resultant reaction does not constitute a second indirect action. Therefore, if he set a timer on the Sabbath to turn on a lamp several hours later on the Sabbath, he violated a Biblical prohibition if he allowed the timer to turn the light on. Even if the timer was set before the Sabbath and he removed the pin and then returned it to its original position, he is guilty. The reason being that we do not take into consideration if the work was going to be done but which action was the cause of it being done. If he set the timer on the Sabbath to turn the bulb on after the Sabbath he violates the prohibition of preparing on Sabbath for after the Sabbath and also the Rabbinical prohibition of improving an instrument. Although it is permitted to set a timer before the Sabbath to turn on lights on the Sabbath, this was not permitted for all works. Some examples will be explained in this book.
It is forbidden on the Sabbath to set the alarm of a battery-operated transistorized alarm clock or a wind-up alarm clock even though the alarm was wound before the Sabbath. The reasons are that he is creating sound with an implement built to produce sound and he is temporarily improving the clock. Even if the clock was prepared before the Sabbath, some authorities prohibit this because making noise on the Sabbath is a degradation of the Sabbath unless it is needed in order to perform a mitzvah such as getting up to make it to the synagogue on time. It is however, permitted to close the alarm if his intention is to stop the noise or an other reason, but not if his intention is to save the batteries. This is because it is comparable to dismantling an implement on condition to rebuild it. However, on Yom-Tov it is permitted unless it is an electric clock plugged into the wall outlet. The reason is that high voltage sparks created by opening and closing the alarm when the alarm is to go off may be hot enough to ignite fuel and is consequently in the category of kindling a fire. If the alarm is disturbing people, and covering it with pillows does not help, he may close it with another person by both of them holding an object and with this object, closing the alarm. This method is called a (פסיק רישיה דלא ניחא ליה בתרי דרבנן) meaning a Rabbinical prohibition upon a Rabbinical prohibition that is being done without having a need for it.
If he changes the window of a digital transistor watch and it will remain changed, he violates the Biblical work of writing. If the window will revert back to the main window then the change was only temporary and he violated the Rabbinical work of writing. It is therefore, advisable not to wear this type of watch on the Sabbath, so that he should not accidentally change the window.
It is permitted on the Sabbath to remove the pin of a timer so that a light or a heater or an air-conditioner or a second timer will not go on if this will prevent much discomfort. This is because the prohibitions of quenching a fire and taking apart an implement pertains only to a fire or a working implement that exists now and not for those that will come into being later. It is also not forbidden to handle the pin because of "muktzah" since it is necessary to do so because of the mitzvah to enjoy the Sabbath.
On Yom-Tov it is permitted to kindle a fire for the needs of Yom- Tov. The Rabbis forbade creating a fire but they permitted kindling a fire from an existing fire. Turning on a bulb on Yom-Tov is creating a fire and not transferring a fire because it is not the heat of the wires that is transferred to the bulb since the wires are not hot. It is the electricity flowing into the bulb that is heating the wire and creating the fire. Therefore, according to those who hold that any fire created on Yom Tov is muktzah, it is forbidden even to shake it, the same holds true for kindling a light bulb. However, he is permitted to use the light emanating from this bulb.
It is permitted to milk a cow on the Sabbath to relieve its pain if the milk is allowed to flow onto the ground and is thus rendered useless. In a situation where he cannot remain on the farm for the entire Sabbath and he cannot milk the cow nor can he get a non-Jew to milk the cow, he is then permitted to attach the milking machine on the Sabbath in an unusual way or with two people where the milk flows to the ground. There are good reasons for a diligent person to be stricter and to attach the machine with two people in an unusual manner.
Plastic hat covers may be worn on the Sabbath in places where there is no "Eruv", if the rain will soak the hat to the extent that he will feel it, or if he is wearing a straw hat. With this condition he is wearing the cover as an attire to protect his body from the rain, and not only to protect his hat.
It is forbidden to open cans on the Sabbath or Yom Tov because through opening the can it becomes an implement, a usable container. There are authorities who permit opening throwaway cans, food bags and cartons, such as tuna fish or sardine cans, pretzel bags and matzo cartons. Screw cap bottles that are attached to a ring around the bottles neck may be opened, providing that in the process no words are broken.
A metal bottle cap that is attached to the bottle by machine forcing the metal on the bottle, and is opened by removing it with force, these caps are permitted to be removed on the Sabbath.
Plastic seals that cover the bottle's cap and neck may be cut in a way that does not break words or figures. It is preferable to prepare the containers before the Sabbath.