The " international " ohm was defined as a mercury column 106.3 cm long of mass 14.4521 grams and 0 �C at the International Electrical Conference 1893 in Chicago.
12.
An actual mercury column reading may be converted to more fundamental units of pressure by multiplying the difference in height between two mercury levels by the density of mercury and the local gravitational acceleration.
13.
The most important advantage of mercury over oil, is that a fair amount of pressure can be maintained within the inert gas system ( equivalent to the height of the mercury column ).
14.
For example, both the old Celsius scale and Fahrenheit scale were originally based on the linear expansion of a narrow mercury column within a limited range of temperature, each using different reference points and scale increments.
15.
There he produced what is usually known as " Berthon's log ", in which the suction produced by the water streaming past the end of a pipe projected below a ship is registered on a mercury column above.
16.
On September 21, 1881 the " Congr�s internationale d'�lectriciens " ( international conference of electricians ) defined a " practical " unit of Ohm for the resistance, based on CGS units, using a mercury column at zero deg.
17.
For some types of thermometer the relationship between the property observed ( e . g ., length of a mercury column ) and temperature, is close to linear, so for most purposes a linear scale is sufficient, without point-by-point calibration.
18.
The tube must be very narrow to minimise the amount of mercury in it the temperature of the tube is not controlled, so it must contain very much less mercury than the bulb to minimise the effect of the temperature of the tube and this makes the reading rather difficult as the narrow mercury column is not very visible.
19.
A " legal " ohm, a reproducible standard, was defined by the international conference of electricians at Paris in 1884 as the resistance of a mercury column of specified weight and 106 cm long; this was a compromise value between the B . A . unit ( equivalent to 104.7 cm ), the Siemens unit ( 100 cm by definition ), and the CGS unit.
20.
The concepts of " total pressure " and " dynamic pressure " arise from Bernoulli's equation and are significant in the study of all fluid flows . ( These two pressures are not pressures in the usual sense-they cannot be measured using an aneroid, Bourdon tube or mercury column . ) To avoid potential ambiguity when referring to pressure in fluid dynamics, many authors use the term " static pressure " to distinguish it from " total pressure " and " dynamic pressure "; the term " static pressure " is identical to the term " pressure ", and can be identified for every point in a fluid flow field.
