The Weston standard cell

Chapter The Weston standard cell

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Other kinds of cells, like the lead-and-acid unit depicted above, can get their chem-ical energy back again. Such a cell is a secondary cell.Primary cells include the ones you usually put in a flashlight, in a transistor radio,and in various other consumer devices. They use dry electrolyte pastes along withmetal electrodes. They go by names such as dry cell, zinc-carbon cell, alkaline cell,and others. Go into a department store and find the panel of batteries, and you’ll seevarious sizes and types of primary cells, such as AAA batteries, D batteries, camera bat-teries, and watch batteries. You should know by now that these things are cells, not truebatteries. This is a good example of a misnomer that has gotten so widespread thatstore clerks might look at you funny if you ask for a couple of cells. You’ll also see realbatteries, such as the little 9-V transistor batteries and the large 6-V lantern batteries.Secondary cells can also be found increasingly in consumer stores. Nickel-cad-mium (Ni-Cd or NICAD) cells are probably the most common. They’re available insome of the same sizes as nonrechargeable dry cells. The most common sizes are AA, C,and D. These cost several times as much as ordinary dry cells, and a charging unit alsocosts a few dollars. But if you take care of them, these rechargeable cells can be usedhundreds of times and will pay for themselves several times over if you use a lot of “bat-teries” in your everyday life.The battery in your car is made from secondary cells connected in series. These cellsrecharge from the alternator or from an outside charging unit. This battery has cells likethe one in Fig. 7-1. It is extremely dangerous to short-circuit the terminals of such a bat-tery, because the acid (sulfuric acid) can “boil” out and burn your skin and eyes.An important note is worth making here: Never short-circuit any cell or battery, be-cause it might burst or explode.The Weston standard cellMost electrochemical cells produce about 1.2 V to 1.8 V of electric potential. Differenttypes vary slightly. A mercury cell has a voltage that is a little bit less than that of azinc-carbon or alkaline cell. The voltage of a cell can also be affected by variables in themanufacturing process. Normally, this is not significant. Most consumer type dry cellscan be assumed to produce 1.5 Vdc.There are certain types of cells whose voltages are predictable and exact. These arecalled standard cells. One example of a standard cell is the Weston cell. It produces1.018 V at room temperature. This cell uses a solution of cadmium sulfate. The positiveelectrode is made from mercury sulfate, and the negative electrode is made using mer-cury and cadmium. The whole device is set up in a container as shown in Fig. 7-2.When properly constructed and used at room temperature, the voltage of the We-ston standard cell is always the same, and this allows it to be used as a dc voltage stan-dard. There are other kinds of standard cells, but the Weston cell is the most common.Storage capacityRecall that the unit of energy is the watt hour (Wh) or the kilowatt hour (kWh). Any elec-trochemical cell or battery has a certain amount of electrical energy that can be gotten120 Cells and batteries