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MATHEMATICAL TERMS 701
exceed the number stated, namely, 8. We state the problem thus 30 : 8 : : 378 : required answer. This is read as follows: As 30 is to 8 so will be 378 to the required answer. We therefore multiply one of the inner amounts by the other (378 X 8) and divide the result by the outer number, thus: 378 X8 = 3,024 -r 30 = 100 3/5 pounds, the answer.
It is always the same, except that in inverse proportion where the required amount will be smaller than the stated amount: in the above case if we wish to ascertain the weight of 20 pounds, knowing the weight of 30, we then would place the smaller number first, thus, 8 : 30 : : 20 : the required answer, which figures out 30 X 20 = 600 and 600-^-8 = 75. You may often find this rule very convenient.
The Positive-Negative Test
At times it may be necessary or convenient to determine which is the positive of a DC circuit before makingfinal connections. One accurate simple test is to cut an ordinary potato in half — a large one if the voltage be 220 — and thrust the wire ends into it the following distances apart:
1 to 3 volts, lA of an inch.
3 to 15 volts, Y% of an inch.
15 to 50 volts, y2 of an inch.
50 to 110 volts, 1 inch.
110 to 220 volts, \y2 inches.
These distances are not intended to be exact, but are efficient for the purpose. When all is readiness close the switch. Soon bubbles will form around the negative, and if the wires are copper, the totato will be stained green around the positive. With low voltages it may require several minutes to get up the effect.
USEFUL EQUIVALENTS
Meters X 39.37 = inches. Meters X 3.281 = feet. Kilometers X 0.621 = miles. Kilometers X 3.281 = feet. Centimeters X .3937 = inches. Millimeters X .03937 = inches. Watts -4 746 = horsepower. Kilowatts X 1.34 = horsepower.