# electrical circuit

An electrical circuit is an assemblage of electrical conductors (usually wires) and components through which current from a power source, such as a battery or generator flows. Components may be connected
one after another (see series circuit)
or side by side (see parallel circuit).
If current is able to flow between two points their connection is a **closed
circuit**; if not, an **open circuit**; and if resistance between them is virtually zero, a **short circuit**: a electric
switch when off is a closed circuit, when on is a short circuit. Short
circuits between the terminals of a power source are dangerous (see circuit
breaker; fuse). See also Kirchhoff's
laws.

## Analysis of electric circuits

The analysis of electric circuits, to find the voltages and currents at
every point, employs two basic laws, illustrated here. The circuit shown
supplies power from a battery (V) to three components (having resistance
R_{1}, R_{2}, R_{3}) through power lines which each
have a resistance (R). First, Kirchoff's "current law" states that at every
junction, electric current neither appears nor vanishes: current is conserved,
thus (center left) I = I_{3} + I_{2}; and so on around the
circuit. Second Kirchoff's "voltage law" states that the total voltage around
any constituent circuit is zero: thus (center right,applying Ohm's law)
V + IR + I_{3}R_{3} + IR = 0; the same applies to the other
two circuit loops. Given the battery voltage and the resistances, the currents
can be calculated. The bottom diagram shows the circuit of an electrical
resistance thermometer, in which changes in the probe resistance, r, are
used to measure the temperature. Contact C is moved along a resistance-wire
(ab) until the detector in CD shows zero current. If the two resistances
R are equal, it is then true that R_{c} + a = b + r; r is read off
from a pre-calibrated scale along the wire ab.