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Q. What is the ohm?

A. The ohm is the unit by which the resistance anything offers to the passage of the electric current is measured.

To overcome one ohm of resistance it will take a current of one ampere at a pressure of one volt.

Q. How are dry batteries measured?

A. Dry batteries are measured as shown at Fig. 87 by means of a simple device which measures the amperage or current output, and which is called an "ammeter." A standard 6''x212" dry cell in good condition should indicate a current strength of from 20 to 25 amperes. The voltage of a dry cell varies from 1.25 to 1.5 volts.

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Fig. 86.-Forms of Primary and Secondary Electric Current Con

ductors.

Q. How is the current strength of storage batteries deteranined?

A. Storage batteries are measured with a voltmeter, which is an instrument similar in construction to an ammeter with the exception that it is designed to register the pressure instead of the quantity of the electric current flowing through it. A single cell of a storage battery is supposed to test 2.2 volts when fully charged, which means that a 2 cell battery will indicate 4.4 volts while a 3 cell battery shows 6.6 volts. The ordinary form of battery testing ammeter should never be used in testing a storage battery, he

AMMETER

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cause its construction is such that it will practically short circuit the battery and it may produce buckling of the plates owing to the rapid discharge of the stored energy. Q. How can current consumption of induction coil be found?

The method of using a simple current indicator to show the amount of electricity used by an induction coil is clearly outlined at Fig. 88. The instrument shown is a low reading ammeter, and is so arranged that the terminal end may be inserted between the switch button and switch lever, while the coil is in operation. The amount of current drawn by each unit can be easily determined by testing out but one unit at a time. This can be easily done by raising the contact screws of the vibrators so that they do not touch the contact on the Fig. 87.Method of Testing Current vibrator blade, except

Strength of Dry Cell With Ammeter. on the unit that is to be tested. The engine is then cranked over by hand and the amount of current consumed by the coil vibrator will be clearly indicated as soon as contact is established at the timer. The ordinary induction coil unit consumes from .5 to 1 ampere, and if the current consumption is greater than this, the tension of the vibrator spring should be lessened so that less current will be necessary to energize the primary winding.

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LESSON NINE

CURRENT PRODUCERS: BATTERIES, DYNAMOS AND

MAGNETOS

Q. What is the simplest method of producing electricity?

A. The simplest method of producing electricity is by the use of a primary or secondary battery which generates electrical energy by the chemical combustion of one of the elements or plates of the battery.

Q. What is the cheapest method of producing electricity?

A. Electricity may be produced at lower cost by magnetic machines which change mechanical power into electricity.

Q. How many classes of batteries are used?

A. Two general classes of chemical current producers are commonly used, these being primary batteries and secondary batteries.

Q. What is a primary battery?

A. A primary battery is one in which electricity is generated by the direct chemical action of the electrolyte on one of the plates composing the cell. A primary cell has positive and negative elements; that attacked by the electrolyte is usually termed the "negative" plate and is made of zinc, either in the form of a plate or a rod. The po.itive element is of some substance that is not affected by the electrolyte and which serves as a collecting member for the electricity generated. In a primary battery electricity is produced at the expense of decomposition of the zinc plate and deterioration of the electrolyte. When a primary battery is exhausted or no longer capable of producing electricity, it must be practically rebuilt in order to extend its life. New electrolyte and zinc must be added to replace that used before the cell will give any more electricity.

Q. What is a secondary battery? A. The secondary battery, commonly called the "storage battery” in America and the "accumulator" in Europe, is a form of chemical current producer in which the elements can be restored to their former condition when exhausted by passing a current of electricity through the battery in a direction opposite to that of the current flow normally given out by the battery. As a battery of this form can only produce electricity after it has been energized or charged by a current from some extraneous source, such as a primary battery or mechanical generator, it is called a "secondary” battery.

Fig. 88.-Outlining Method of Determining Current Consumption of

Induction Coil by Using Low Reading Ammeter.

Q. What is a dry battery ?

A. A dry battery is the form of primary cell in which the liquid electrolyte is replaced by some form of jelly or other substance to absorb the liquid so it cannot splash out while the automobile is in operation.

Q. Describe construction of typical dry cell.

A. A part sectional view of the conventional form of dry cell is outlined at Fig. 89. The container or shell is made of zinc and

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Fig. 89.-Sectional View of Typical American Dry Cell.

serves as the negative element, while a stick of carbon placed in the center of the zinc shell and kept from contact with the metal serves as a positive element. The electrolyte or exciting material is absorbed by blotting paper, several layers of which are closely wound

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