Metrication in the United States There are no U.S. customary units for electric current, potential difference, or charge since these concepts were developed after the international adoption of metric in science. The metric units ampere, volt, ohm and coulomb are the only units used. The SI term hertz has replaced the term "cycles per second" as a unit of frequency. Energy is often measured in watt hours, BTUs, therms or calories rather than the SI joule. Residential and commercial electrical energy use is metered and billed in kilowatt hours, kW·h, as in most metric countries. Force measurements regularly use the pound instead of the SI
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