Power balance

Structure borrowed from the Ampere current balance

The current balance was developed by André-Marie Ampère during experiments on electric current . It consists of a solid ladder on which a U-shaped wire bracket hangs, which can move freely like a pendulum. This device can be used to carry out experiments that show the interaction of two conductors through which current flows.

Ampère was able to show that parallel electrical conductors attract each other when current flows through them and that the current flows in the same direction in both conductors and repel each other when the current flow is opposite. He suspected that a magnetic field arises around the current-carrying conductors.

In further experiments, Ampère discovers that an electrical conductor that is wound into a coil behaves like a bar magnet . He used a volta element , a simple chemical battery, as a voltage source .

Kelvin's current balance

Kelvin's current balance

Lord Kelvin developed a new type of electric balance in 1882. This consists of a beam balance with a coil on each of the two ends of the beam. Two more immobile coils are attached above the two movable coils. The four coils are connected in series so that the same current flows through each coil, in such a way that the coils attract each other on one side of the scale and repel each other on the other. From the weight that is required to keep the balance in balance, the force between the coils can be calculated depending on the current strength and this makes it possible to calibrate current strengths in amperes .

Kelvin power balances were manufactured by White of Glasgow and exported to the United States by James W. Queen of Philadelphia, where they were sold for $ 200 apiece in 1889.

Current balance in the physics experiment

Typical experimental setup

In physics lessons, a current balance structure is a popular means of experimental investigation of the magnetic flux density B. A field coil with current generates a constant magnetic field that is guided through a soft magnetic material . A U-shaped conductor bracket is placed in the magnetic field and a known (measured) current flows through it. An electronic scale measures the change in force compared to the de-energized state, in which only the weight force acts. By changing the current flow and different lengths of ladder bracket, their relationship with the force is shown:

${\displaystyle F\sim I}$ and ${\displaystyle F\sim l}$

This determines the magnetic flux density with the following relationship:

${\displaystyle {\frac {F}{I\cdot l}}=konstant}$

${\displaystyle {\frac {F}{I\cdot l}}=B}$

Web links

• Description of the Ampère current balance - Werner B. Schneider (Ed.), Paths in Physics Didactics Volume 1 (PDF file; 605 kB)
• Force measurement with current balance (at LEIFIphysik.de)
• Current Balance , Kelvin's current balance (English)