How fast is greased lightning

Time is one of the so-called base units within the SI system, and so is length. Whereas volume can be expressed in terms of a length (for example, a cube has a volume l3 and side of area l2), we cannot define length in terms of something simpler. Similarly, whereas a velocity is a length per unit time, we cannot express time in terms of something simpler. In fact, just as compounds are made up of elements, so all scientific units are made up from seven base units length, time, mass, temperature, current, amount of material and luminous intensity. 

The SI unit of time t is the second. The second was originally defined as 1/86 400th part of a mean solar day. This definition is 

There are seven base SI units length, time, mass, temperature, current, luminous intensity and amount of material. 

In a similar way, the Systeme Internationale has defined other common physicochemical variables. The SI unit of temperature T is the kelvin. We define the kelvin as 1/273.16th part of the thermodynamic temperature difference between absolute zero (see Section 1.4) and the triple point of water, i.e. the temperature at which liquid water is at equilibrium with solid water (ice) and gaseous water (steam) provided that the pressure is 610 Pa. 

The SI unit of current I is the ampere (A). An ampere was first defined as the current flowing when a charge of 1 C (coulomb) passed per second through a perfect (i.e. resistance-free) conductor. The SI definition is more rigorous the ampbre is that constant current which, if maintained in two parallel conductors (each of negligible resistance) and placed in vacuo 1 m apart, produces a force between of exactly 2 x 10-7 N per metre of length . We will not employ this latter definition. 

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