DC Electrics - DC Motors — Page 117, Lesson 118

I want to walk you through a new topic: electromagnetic indicators. These are devices used on aircraft panels to show system status, and they replaced an older, simpler type of indicator — the filament lamp indicator. So, the electromagnetic indicator was introduced as a replacement for the simple filament lamp indicator. Instead of just a light bulb that glows, we now have a more sophisticated visual indicator that uses an electromagnetic mechanism. There are two common types in use: the doll’s eye indicator and the prism indicator. Both are illustrated in Figure 7.11. Let me explain what these look like. The "doll’s eye" indicator gets its name from its appearance — it has a small, round, eye-like element that changes position to show a state, such as "OPEN" or some other condition. The "prism" indicator uses a small prism that rotates or moves to present a different coloured face or symbol to the viewer. Both of these indicators give a clear pictorial presentation. But the book tells us that their usefulness is further improved by painting "flow lines" on the appropriate panels. These flow lines are painted lines that interconnect the indicators with the system control switches, essential indicators, and warning lights. So, when you look at the panel, you see a visual path — a flow line — that connects the switch you operate to the indicator that shows the result, and to any related warning lights. This helps the pilot quickly understand the relationship between controls and indications without having to search the panel. So, to summarise: electromagnetic indicators — either the doll’s eye type or the prism type — replaced simple filament lamps. They give a clear visual state indication, and their effectiveness is enhanced by painting flow lines on the panel to link them logically with switches and other indicators.