Voltage power and current, where 1 volt = 1 watt (of power) per 1 ampere (of current). This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by , but more often simply as "V", for instance in the context of Ohm's or Kirchhoff's circuit laws. Electric potential differences between points can be caused by electric charge, by electric current through a magnetic field, by time-varying magnetic fields, or some combination of these three. A voltmeter can be used to measure the voltage (or potential difference) between two points in a
Negative resistance be defined, but the resistance is not constant; it varies with the voltage or current through the device. The resistance of such a nonlinear device can be defined in two ways, which are equal for ohmic resistances: Negative resistance, like positive resistance, is measured in ohms. Conductance is the reciprocal of resistance. It is measured in siemens (formerly "mho") which is the conductance of a resistor with a resistance of one ohm. Each type of resistance defined above has a corresponding conductance It can be seen that the conductance has the same sign as its corresponding resistance: a negative resistance