Referring to the switchboard of Fig. 259, the three switches and the ammeter comprise the necessary accessories. The starting switch is so arranged that by pressing the lever a current of electricity from the storage battery is sent through the dynamo. The dynamo acting as a motor starts the engine. When the engine has attained its proper speed its function as a dynamo overcomes the current pressure from the battery and sends electricity into the cells to restore the expended energy, or if so desired the current may be used directly from the dynamo for any household purpose. The box enclosing the switch contains a magnetic circuit-breaker so constructed that when the battery is completely charged the switch automatically releases its contact and stops the engine.
The “stopping switch” at the right of the board and the “switch for light and circuit” on the left are used respectively for stopping the engine and for opening and closing the house circuits.
The meter performs a multiple function, in that it shows at any time the condition of charge in the storage battery, the rate at which current is entering or leaving the battery and also acts to stop the engine when the battery is charged. At any time the pointer reaches the mark indicated in the picture, the ignition circuit is automatically broken and the engine stops. The fuses on the board in this case perform the same function as those already described.
Storage Batteries.
—These batteries have already been mentioned as secondary batteries. They are sometimes called electric accumulators. The electricity is stored or accumulated, not by reason of the destruction of an electrode as in the primary cell but by the chemical change that takes place in the plates as the charging current is sent through the cell. When the battery is discharged, the current from the dynamo is sent through the battery circuit in the reverse direction to that of the discharge and the plates are restored to their original condition. The action that takes place in charging and discharging is due to chemical changes that take place in the plates and also in the solution or electrolyte in which the plates are immersed.
There are two types of storage batteries, those made of lead plates immersed in an acid electrolyte and the Edison battery which is composed of iron-nickel cells immersed in a caustic potash electrolyte. The former type is most commonly used and is the one to be described.
The lead-plate cell illustrated in Fig. 262 shows all of the parts of a working element. The plates are made in the form of lead grids which when filled to suit the requirements of their action, form the positive and negative electrodes. The negative plates are filled with finely divided metallic lead which when charged are slate gray in color. The positive plates are filled with lead oxide. When charged they are chocolate brown in color. In the figure there are three positive and four negative plates which together form the element, then with their separators are placed in a solution of sulphuric acid electrolyte. The separators are thin pieces of wood and perforated rubber plates that keep the positive and negative plates from touching each other and keep in place the disintegration produced by the electro-chemical action of the cell.
The unit of electric capacity in batteries is the ampere-hour. The cell illustrated will accumulate 80 ampere-hours of energy. It will discharge an ampere of current for 80 hours. If desired it may be discharged at the rate of two amperes for 40 hours, or four amperes for 20 hours, or at any other rate of amperes and hours, the product of which is 80. The number of ampere-hours a cell will accumulate will depend on the area of the positive and negative plates; large cells will store a greater number of ampere-hours than those of small size.
The cells, no matter what size, give an average electric pressure of 2 volts.
The plates are joined by heavy plate-straps connecting all of the positives on one end and all of the negative kind on the opposite end. To insure rigidity the two sets are secured to the rubber cover by locknuts. In this cell the plates are suspended from the cover. The plate terminals are made of heavy lead connectors that when formed into a battery are joined together with lead bolts and nuts.