Current through series resistances:

Have you ever used those tiny holiday lights which come in strings? If one bulb burns out, the whole set of bulbs stops lighting; then you have to find out which bulb is burnt, and replace it to get the lights working again. Each bulb works with some10 V; there are around a dozen bulbs in the string. You plug in whole bunch and the 120-V utility mains drive the right amount of current through each bulb.

In the series circuit, such as a string of light bulbs shown in figure given below, the current at any given point is the same as the current at any other point. The ammeter, A, is shown in line between the two of the bulbs. If it were moved to the other place along the current path, it would indicate same current. This is true in any series direct current circuit, no matter what the components are and regardless of whether or not they all are having same resistance.

If the bulbs in shown in the figure were of different resistances, some of them would consume more power than others. In case one of the bulbs in the Figure burns out, and its socket is then shorted out instead of filled with the replacement bulb, the current through the whole chain will increase, as the overall resistance of the string would go down. This would force the remaining bulbs to carry too much current. Another bulb would burn out before long. If it were replaced with a short circuit, then the current

Figure--   Light bulbs in series. An ammeter, A is placed in circuit to measure current.

would be increased further. A 3rd bulb would probably blow out almost right away after string was plugged in.