Immagini

DESCRIPTION

Exhibit 179 is a Siemens standard one Ohm resistance guaranteed correct at a temperature of 16,0 degrees Centigrade. It has the form of a small coil enclosed and sealed within a wood cylinder with an hard rubber cover which has an opening for a thermometer for temperature control. The contacts are two large rectangular metallic bars of negligible resistance which have both screw contacts as well as pins for immersion in mercury.

At that time the standard was defined as the resistance of a column of pure mercury of cross section 1 mm squared and one meter long at a temperature of zero degrees Centigrade.

In the circuit of design-1 based on the Kelvin bridge, we have CD is a circular bar of diameter d and unknown resistance, r1,r2,r3,and r4 are constantina wires of equal resistance, G is a galvanometer to indicate equal voltages at terminals, M is the resistance of the standard X and Y are two contacts on the bar separated by distance L, the specific resistance is then FR06: [ {\rm M\pi\frac{D^2}{4L}} ]..

To be reliable the current due to closing K1 must last little time to avoiding heating the standard resistance.

ISCRIZIONI

1. Inciso sul coperchio di ebanite: "RICHTIG BEI 16.0 C."¶
   "APRIL 1879"¶
   "2292"
2. Inciso sulle sbarre: "Hg 1M, 1 |_| M m, 0{\textdegree}C"¶
   "SIEMENS & HALSKE Nr 2292"

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