How did Einthoven's string galvanometer work?
The string galvanometer was born from a physical problem: how to measure the extremely weak electrical currents—on the order of millivolts and microamperes—generated by the beating heart. Earlier instruments, like the capillary electrometer, were too sluggish. My solution was to suspend an extremely fine, silver-coated quartz fiber—only a few microns thick—between the poles of a powerful electromagnet. When the heart's electrical current passed through this fiber, it experienced a Lorentz force, causing it to deflect sideways. I projected a bright light onto the fiber and recorded its shadow on a moving photographic plate, creating a continuous, magnified trace of the deflections. The key was the fiber's minimal inertia, which allowed it to respond almost instantaneously to rapid electrical changes. This instrument transformed physiology by making invisible electrical events visible and measurable. In physiology, as in physics, the instrument defines the observation.
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