Amazing invention of Sir William Thomson’s mirror galvanometer turned almost invisible currents into visible light, allowing engineers to accurately test the insulation resistance of long telegraph cables and other high‑resistance circuits.
What is being measured:
• The setup measures insulation resistance of a cable or unknown high resistance by sending a small DC voltage from a battery through a Wheatstone bridge.
• By adjusting the known resistances in the bridge until the galvanometer shows no deflection, the operator can read the insulation resistance from the bridge settings.
How the mirror galvanometer works
• Thomson’s instrument uses a tiny magnet and mirror suspended on a fine fibre inside a coil; current in the coil twists the magnet, rotating the mirror.
• A lamp projects a narrow beam onto this mirror, and the reflected spot of light moves across a distant scale, acting as an ultra‑sensitive “pointer” for very weak currents.
Why it was important
• The mirror galvanometer was sensitive enough to detect currents up to about a thousand times weaker than older receiving instruments, crucial for long submarine cables where signals arrived very faint.
• This technique helped monitor cable condition, locate faults, and made reliable transatlantic telegraphy possible, cementing Thomson’s role (later Lord Kelvin) in the history of electrical engineering.