Dolter System (Surface Contact Current Collection)

The Dolter System

(Surface Contact Current Collection)

The surface contact method of tramway current collection originated in France and some fifty miles of lines were laid in Tours, Lorient and Paris with moderate success. The intention was to find a method of operation not requiring overhead wiring, as this often raised objections on aesthetic grounds. In central London and – for a time – in Bournemouth and Blackpool, this was overcome by the conduit system which was, however, expensive to install and maintain and the surface contact was considered a cheaper alternative. In this country very few systems were equipped in this way and only a few remained in service for long periods as the following table shows:

In the cases of Hastings and Mexborough, the adjacent systems over which the cars had running powers employed overhead collection, and the trams had therefore to be fitted both types. Other schemes were planned for Oxford and Folkestone but never put into practice.

The Dolter installation at Torquay operated as follows. The 550 volts supply was split into sections of feeder cables which were laid under the road surface in the centre line of the tracks and into these at regular intervals were fitted the Dolter pots. These were of a ceramic insulated material with their caps just flush with the road surface. Into their tops were set two metal pads which were normally dead electrically.

Below the tram were suspended two skids, polarised north and south by coils energised either from the car’s operating current or from a bank of batteries carried on the vehicle. This latter supply was to activate the first contact plate when starting and then cut out. As the skids passed over a Dolter box surface, the two metal pads were magnetised and this attracted a pivoted arm inside the ceramic box. On rising, this closed a set of contacts and the main current then flowed up through the plate and via the skids to the tram controller and motors. When the tram had passed, the magnetic field ceased and the pivoted arm dropped by gravity, cutting off the supply and the road surface plate became inert. There was also a smaller skid, following the main pair and when this passed over a plate which had remained live it caused a bell to ring on the tram to warn the crew.

Corrosion from the sea air affected the action of the pivot arm with unfortunate results. Either it remained up, dispensing up to 550 volts to anything which which it came into contact, or it stayed stubbonly down and the tram received no supply. The immediate methods of dealing with both these situations have already been described but these were only temporary remedies and a follow-up gang had to work on the faulty plates and replace or service them. The Dolter boxes were designed to be lifted out for maintenance and were clipped to the main supply cable.