is a widely known concept in electrical study. Of the three units of measure involved, only current (ampere) is an independent unit. Voltage and resistance units are dependent on current units, as defined by Ohm's law.
It is believed that each of independent units of measure will be defined in terms of the other four independent units eventually. Length (meter) and time (second) are already connected this way. If an accurate time base is available, then a length standard can be reproduced without a meter bar artifact, using the known constant speed of light. Lesser known is the relationship between the luminance (candela) and current (ampere). The candela is defined in terms of the watt, which in turn is derived from the ampere.
Non-commercial measurement details used to be academic curiosities. The development of standards follows the needs of technology. As a r esult, some units of measure have much more resolution than others. The second is reproducible to 1 part in 10>14. As it became possible to measure time more precisely, solar time, believed to be a constant, proved to be very slightly irregular. This resulted in leap second adjustments to keep UTC (universal time coordinate) synchronized with solar time. The candela standard is difficult to recreate. An incandescent bulb design must be used as a secondary standard, a transfer standard. These special candela standard bulbs recreate the candela when a sp ecific amount of current is applied. Luminance (candela) can only be reproduced to 5 % of reading despite having sensors that have accuracies of +/– 50 parts per million (0.005 %) precision. This is due to the standard not being accurately reproducible.
Temperature (kelvin) is defined by agreed fixed points. These points are defined by the state changes of nearly pure materials, generally as they move from liquid to solid. Between these fixed points, Standard Platinum Resistance Thermometers (SPRTs) are used to interpolate temperature values. This mosaic of approaches produces measurement uncertainty which is not uniform over the entire range of temperature measurement. Temperature measurement is coordinated by the International Practical Temperature Scale, maintained by the BIPM.
Ex. 2. Answer the following questions