### Video Transcript

Which of the following SI units is
defined by reference to the properties of a unique, standard object held at the
International Bureau of Weights and Measures? (A) The meter, (B) the kilogram,
(C) the mole, (D) the candela, (E) the steradian.

The question has given us five
different SI units, and it’s asking us to identify which of them is defined by
reference to the properties of a unique standard object. Let’s begin by recalling what
physical quantity each of these units is used to measure. In option (A), we have the meter,
and we can recall that this is the SI unit for length or distance. Option (B) is the kilogram, which
is the SI unit for the quantity mass. Option (C) is the mole, which is
the SI unit for the amount of substance or the number of equivalent parts that
something consists of. It’s worth noting that whenever
we’re talking about the number of moles of a substance, the chances are we’re
talking about the number of atoms or molecules.

Then in option (D), we have the
candela and this is the SI unit for luminous intensity. Lastly in option (E), we have the
steradian and this is the SI unit of solid angle, which is used in three-dimensional
geometry.

Now, let’s think about how each of
these units is defined. The definition of the meter is
defined by a measurement of a naturally occurring physical phenomenon. Previously, this definition was
based on the light emitted from a particular kind of atom. The definition has since been
updated: the distance traveled by light in vacuum in a particular interval of
time. Since this definition is based on
the speed of light in vacuum, then our definition of the meter is still based on a
naturally occurring physical phenomenon. We know then that the meter is not
defined by reference to the properties of a unique standard object, so we can
eliminate answer option (A).

Let’s move on then and look at
answer option (B). The kilogram was originally defined
as the mass of one liter of water. However, in 1889, a specially
designed object was made and this definition was updated. The kilogram was then redefined to
be equal to the mass of this object, which was a cylinder made of platinum–iridium
alloy. This unique object was known as the
prototype kilogram and was kept at the International Bureau of Weights and
Measures. We can see then that this
definition of the kilogram is indeed based on the properties of a unique standard
object. And so option (B) is looking like
it may well be our answer.

Let’s quickly check out the
remaining three options though just to be sure, starting with option (C) the
mole. One mole is defined to be exactly
equal to 6.02214076 times 10 to the 23 particles or parts of substance. This is a mathematical definition
and not based on any unique standard object. So we can eliminate answer option
(C). The SI unit in option (D), the
candela, is defined as the brightness of a light source with certain properties. So in a sense we could say that
this definition is based on an object, but it’s certainly not a unique object. There is no standard unique one
candela object kept at the International Bureau of Weights and Measures. So we can eliminate answer option
(D).

The last SI unit to consider is the
one in option (E), the steradian. The definition of the steradian is
a mathematical one based on the geometry of a sphere. This definition exists in the
abstract sense, and there is no standard object used. That means then that we can
eliminate answer option (E). We have found that neither the
meter, the mole, the candela, or the steradian is defined by reference to the
properties of a unique standard object. The only one of these units that is
defined by reference to the properties of such an object is the kilogram, which is
the SI base unit for mass.

Our answer then is that the SI unit
which is defined by reference to the properties of a unique standard object held at
the International Bureau of Weights and Measures is given in option (B) the
kilogram.