The Slab-Desynn System

In addition to the cyclic error present in the basic and micro-type systems, small errors also arise due to friction set up by the contact arms having to move over a considerable surface of resistance wire. Although such errors can be reduced by providing a good contact material and by burnishing the resistance wire surface, the cyclic error is still undesirable in certain measurements.

Figure 4.3.11: Slab-Desynn transmitter

A solution to this problem was brought about by modifying the basic system so as to change its three sawtooth waveforms into sinusoidal waves, the instantaneous sum of which is always zero. The transmitter so developed is shown schematically in Figure 4.3.11, from which it will be noted that the resistor and contact arms have, as far as electrical connections are concerned, virtually changed places with each other. The resistor is now wound on a slab former, hence the term `slab-Desynn', and is connected to the direct-current supply, while the contact arms themselves provide the three potential tapping points for the indicator stator.

The three contact arms are insulated from each other and pivoted over the centre of the slab, and are each connected to a slip ring. Spring-finger brushes bear against these slip rings and convey the output currents to the stator coils. Movement of the mechanical element is transmitted to the brushes via a gearing system.