The piezometer is then introduced into the cylinder already filled with the piezometric liquid and, once closed, filling is completed through the funnel provided for the purpose.
Initially, the piezometric liquid is at atmospheric pressure and the manometer is full of air; when compressed, also the air in the manometer is compressed and the pressure is derived from the decrease in volume.
Three scales are etched on the silver-plated plate: two of them, one for measuring miscellaneousations in volume of the liquid under examination and the other for the pressures, are graduated in millimeters; the third, for measuring temperature, is graduated from 0 to 65° Celsius.
However, it is to be kept in mind that the measurement, as Oersted himself pointed out, is affected by a systematic error:
The decrease in volume undergone by the tank, which is subject to the same pressure, both inside and outside, is not taken into account, but the correction is slight.
A brass wrench is provided with the instrument; on the one hand this allows it to be opened at the top by unscrewing the brass cylinder with the piston in order to fill it with piezometric liquid and for introducing the piezometer; on the other hand, it can be used to unscrew the head of the piston, represented by a brass plate, for substitution of the leather gasket of the piston.