The exchange had some advanced features for the time, including multi-frequency (MF) tone dialling as an option opposed to pulse dialling, and no post-dialling delay for own exchange calls. It also had the ability to detect a switching failure and automatically attempt a repeat dial. Any repeat attempts were logged to the teleprinter. It also had a test console, which monitored all calls on a digital edge-lit display. Another display gave a visual indication of the traffic flowing through the exchange, named the Hubblemeter after the instigator Ray Hubble. Occasionally the call trace did not work, but the engineers worked out a way of manually tracing a call. What they did was to buy a little compass and glue a piece of magnetic ferrite on the side to pull the compass needle away from north. They would then run this compass along outside of the reed relays, and when a relay was operated the needle would move back to north. This was repeated over several sets of the switching path until the trace was complete.