more than official

With the two new scenes I added in tonight, With Fate Conspire passed the 150,000 word mark. (150,975 words, to be precise.)

Nothing next to the bricks of epic fantasy, of course -- but more than long enough for me. Unfortunately for that sentiment, I have four more scenes to add before this revision is done. Please, God, don't let this book balloon all the way up to 160K . . . .

for my science friends

I'm not sure how to phrase this best, but -- at what point in history did we start to develop actual, workable "detection" devices? I'm thinking of things along the lines of a Geiger counter, but it doesn't have to be a radiation detector; just a device to measure anything not visible to the eye. Wikipedia claims Gauss invented an early magnetometer in 1833, but the claim consists of three not terribly informative sentences, and the article on Gauss himself just says he developed a "method" for measuring magnetism, without specifying what it was.

Basically, Fate may or may not end up including a device for the measuring of a particular substance/effect/force/whatever, and I'm trying to figure out how much the concept of such a thing existed by 1884. (The question of how this thing works can be dealt with separately, if I decide to include it.)

Any historians of science able to answer that one for me?