Inside the torsion-head is a commutator for automatically reversing the current, so that readings may be taken on each side of zero, and the arrangement is such that when the torsion-head is exactly at zero the current is interrupted.
The position of the reversing link is altered by means of a cord, passing over two pulleys, fixed respectively in the engine-house and on the derrick.
Units in the case illustrated in the figure), an operation which is performed by simply reversing, the direction of the maximum magnetizing current a few times.
The reversing key K having been put over to the left side, the short-circuit key S is suddenly opened; this inserts the resistance R, which has been suitably adjusted before hand, and thus reduces the current and therefore the magnetizing force to a known value.
To continue the process, the key K is turned over to the right-hand side, and then, while S is open, is turned back, thereby not only reversing the direction of the current, but diminishing its strength by an amount depending upon the previous adjustment of R2.