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The BIPM watt balance: general concept
Summary
The BIPM watt balance: general concept
The principle of the watt balance
Present status
Coil suspension and electrostatic motor
Current source and voltage reference
Magnet
Magnetic field alignment
Optical systems to measure coil displacements
Dynamic alignment mechanism
Vibration isolation
Local gravitational acceleration cartography
Cryogenic watt balance: general concept
Cryogenic watt balance: present status
BIPM collaborators on the project
Recent publications
Director's Report 2012
Direct access

The distinguishing characteristic of the BIPM watt balance with respect to all other watt balances world-wide is that it carries out both measurement phases (weighing and moving experiments) simultaneously. The great advantage of this simultaneous measurement is to guarantee that the geometric properties of the coil, the coil orientation and the magnetic field, acting in both phases are strictly the same. The disadvantage is the appearance of an undesirable voltage drop in the coil, which is a result of the current flow and the non-zero resistance of the coil. This undesirable voltage drop needs to be separated from the induced voltage with an uncertainty of 1 part in 108. Such high accuracy is difficult to achieve at room temperature due to the resistance changes in the coil as a consequence of its temperature coefficient. One possibility is to ultimately develop a cryogenic watt balance with a superconducting coil (with zero resistance), moving in a permanent magnet in a cryogenic environment.

The BIPM watt balance

The BIPM watt balance