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44 RICHARDSON'S BLUEBOOK Of PROJECTION
73810
=100 ohms, approximately. This value is of
731 course less than that of the smaller resistor, since a parallel path has been added around that smaller resistor, permitting still more current to flow. The total current flowing is: 0.909 amperes through 122 ohms; 0.180 amperes through 610 ohms; 1.089 amperes altogether.
(9) Students who duplicate these calculations will note that the figures are not carried out to the last possible fraction, but are only approximately accurate. The projectionist will never, in his practical work, have occasion to carry his figuring to absolute arithmetical accuracy, but only to approximate accuracy in which small fractions left over are disregarded.
(10) Where two resistors in parallel are of equal value it is not necessary to use the formula cited above, but only to divide by 2. Thus, if the resistors of the parallel circuit of Fig. 11 were each of 100 ohms value, their net effect in the circuit would be that of one resistor of 50 ohms. If there are three equal resistors divide by 3 ; if there are four, divide by 4, etc. It is only where parallel resistors are of unequal values that the formula is needed.
( 1 1 ) In the series circuit of Fig. 1 1 all current that flows through R-3 must also flow through R-4, and vice versa. Hence, connecting resistances in series has the effect of adding to the amount of resistance an electron must overcome in passing around the circuit. No formula is necessary for resistors in series. Their values add. The total resistance in the series circuit of Fig. 11 is 122 + 244 or 366 ohms. This at 220 volts gives a current of 220 -f 366 or about 0.6 ampere. The same current, of course flows through both resistances, which fact involves the very important phenomenon of "voltage drop."
(12) Since the current in the top resistor of the series circuit is 0.6 amperes and the resistance is 122 ohms, the voltage acting across that resistance must, by Ohm's Law, be 122 x 0.6, or 73.2 volts. That is all the voltage that can possibly act across R-3 ; if there were a greater voltage across that resistance the current flow through it