| by Siemens and | 105.98 |
| | | | Streker | |
|F. E. Dorn | 1889 | Damping of a magnet | | 106.24 |
|Wild | 1883 | Damping of a magnet | | 106.03 |
|L. V. Lorenz | 1885 | Lorenz method | | 105.93 |
+-----------------+------+----------------------+-------------------+---------+

For a critical discussion of the methods which have been adopted in
the absolute determination of the resistivity of mercury, and the
value of the British Association unit of resistance, the reader may be
referred to the _British Association Reports_ for 1890 and 1892
(_Report of Electrical Standards Committee_), and to the
_Electrician_, 25, p. 456, and 29, p. 462. A discussion of the
relative value of the results obtained between 1882 and 1890 was given
by R. T. Glazebrook in a paper presented to the British Association at
Leeds, 1890.

_Resistivity of Copper._--In connexion with electro-technical work the
determination of the conductivity or resistivity values of annealed
and hard-drawn copper wire at standard temperatures is a very
important matter. Matthiessen devoted considerable attention to this
subject between the years 1860 and 1864 (see _Phil. Trans._, 1860, p.
150), and since that time much additional work has been carried out.
Matthiessen's value, known as _Matthiessen's Standard_, for the
mass-resistivity of pure hard-drawn copper wire, is the resistance of
a wire of pure hard-drawn copper one metre long and weighing one
gramme, and this is equal to 0.14493 international ohms at 0 deg. C.
For many purposes it is more convenient to express temperature in
Fahrenheit degrees, and the recommendation of the 1899 committee on
copper conductors[8] is as follows:--"Matthiessen's standard for
hard-drawn conductivity commercial copper shall be considered to be
the resistance of a wire of pure hard-drawn copper one metre long,
weighing one gramme which at 60 deg. F. is 0.153858 international
ohms." Matthiessen also measured the mass-resistivity of annealed
copper, and found that its conductivity is greater than that of
hard-drawn copper by about 2.25% to 2.5% As annealed copper may vary
considerably in its state of annealing, and is always somewhat
hardened by bending and winding, it is found