It is odd to think that the resistance-wire strain gauge could have been put to use some 40 years ago, about the time of the first filament-wire lamp and the thermionic valve. The physical principles involved were already known. Kelvin had measured the resistance of wires in tension, there were galvanometers in plenty, and even crude valve amplifiers existed. It only really remained for someone to put two and two together. Instead we had to wait until the necessity of stressed-skin construction forced the hand of the engineer into seeking a more effective method of strain analysis than the old empirical ways.