A working knowledge of these simple formulas you will find very helpful when servicing Radio receivers. In most cases when a resistance in a receiver or power supply device burns out it is as a rule a simple matter to find out the value of the resistance as very often the value of the resistance is marked on it, but in other cases it will be necessary to use your knowledge of Ohm's and Kirchhoff's Laws.
For example, in Fig. 13 we have a Radio arrangement of resistances as used in power supply devices. The resistance R between the 180V and 90V terminals is the one that usually burns out. If we had to replace this resistance and we did not know its resistance value, how could we calculate it?
Fig. 13A voltage divider.
The easiest way, if a duplicate power supply device was available, would be to measure the value of the good resistance with an Ohmmeter, or we might remove the good resistance R temporarily and measure its resistance by applying a certain voltage to it and measuring its current and then using Ohm's LawResistance equals the voltage divided by the current.
In case that a good resistance is not available, then procure a variable wire-wound resistance and use it in place of R. Connect a voltmeter between the 90V tap and the B minus terminal then vary the resistance until the correct voltage is shown by the voltmeter. After this has been done, the variable resistance will have the correct value of the resistor R which was burned out, it can be removed and measured as explained above for measuring a good resistance and the proper substitution made.