=290. Energy and Power of a Storage Cell.=--In a storage cell, the electrical energy of the charging current is transformed into _chemical_ energy by the action of electrolysis. It is this chemical energy that is transformed into the energy of the electric current when the cell is discharged. The _capacity_ of storage cells is rated in "ampere hours," a 40 ampere hour cell being capable of producing a current of 1 ampere for 40 hours, or 5 amperes for 8 hours, etc. The production and extensive use of electric currents have made necessary accurate methods for measuring the _energy_ and _power_ of these currents. To illustrate how this is accomplished, let us imagine an electric circuit as represented in Fig. 268. Here four storage cells in series have an E.M.F. of 8 volts and in accordance with Ohm's law produce a current of 2 amperes through a resistance in the circuit of 4 ohms. Now the work done or energy expended by the current in passing through the resistance between the points _M_ and _N depends_ upon three factors (1) the E.M.F. or _potential difference_; (2) the _current intensity_ and (3) the _time_. The energy is measured by their product. That is, _electrical energy_ = _potential difference_ × _current intensity_ × _time_. This represents the electrical energy in _joules_, or Joules = volts × amperes × seconds, or _j_ = _E_ × _I_ × _t_. In the circuit represented in Fig. 268 the energy expended between the points _M_ and _N_ in 1 minute (60 seconds) is 8 × 2 × 60 = 960 joules. =291. Electric Power.=--Since power refers to the _time rate_ at which work is done or energy expended, it may be computed by dividing the electrical energy by the time, or the _electrical power_ = _volts_ × _amperes_. The power of 1 joule per second is called a _watt_. Therefore, Watts = volts × amperes, or Watts = _E_ × _I_. Other units of power are the _kilowatt_ = 1000 watts and the _horse-power_ = 746 watts. In the example given in Art. 290 the power of the current is 8 × 2 = 16 watts, or if the energy of the current expended between the joints _M_ and _N_ were converted into mechanical horse-power it would equal 16/746 of a horse-power. Electrical energy is usually sold by the _kilowatt-hour_, or the amount of electrical energy that would exert a power of 1000 watts for one hour, or of 100 watts for 10 hours, or of 50 watts for 20 hours, etc. Important Topics 1. The storage battery, its construction, electrolyte, action, uses, advantages, disadvantages. 2. Electric energy, unit value, how computed? 3. Electric power, three units, value, how computed, how sold? Exercises 1. In what three respects are voltaic and storage cells alike? In what two ways different? 2. Name the four advantages of storage cells in the order of their importance. Give your reasons for choosing this order. 3. Why are dry cells more suitable for operating a door-bell circuit, than a storage battery? Give two reasons.