JF Ptak Science Books  Post 1749  [Part of the Strange Things in the Sky department.]

The Death Ray is a long-discussed idea, extending back as far to Archimedes at least–discussed, attempted, abandoned and dismissed.  But as a matter of fact, the thing was actually invented, and deployed, though not int he sense of an EM weapon, or LRAD/ultrasonic, or Teller x-ray laser, or even a Wellsian heat ray (below).

The “Death Ray” made its appearance in the 1880’s, but not in the normal sense of what we would today think of as a “weapon”–this death ray could locate the enemy hidden miles from the front, or pick out ships at sea far from shore, and so on, removing stealth capacity, making it possible for these elements to be identified as targets, and then possibly removed, though not by the ray itself. 

This “death ray” was the search light.  In the 1880’s when the technology of electric lighting was still in its first practicable decade, the idea of being able to focus a beam of light from a lantern source hauled on a single-mule carriage and powered by an on-board battery, small steam engine  and Gramme dynamo was a spectacular. achievement. 

[This image appeared in the Scientific American in 1886 and features what is probably a one-foot diameter mirror, making it capable of illuminating an object up to about a mile away.  Something with a three-foot diameter could work its magic on object up to four miles away.]

This defensive/offensive weapon/device was very soon afterwards made into a trickle-down appliance that was placed into commercial use almost immediately.  The standard use of course would be upgrading lighthouses, but one special use was using a large mirror in a device to project an advertisement on the clouds in a city–ads in the sky.

[Source, La Nature, 1894; also reprinted in Scientific American in the same year.]

Such a device was used experimentally at the Columbian Exposition in Chicago for the World’s Fair of 1893, flashing the daily attendance on the clouds.  I’m not sure why the greater revenue-generating employment of this technology took another year to develop.  And so “The Death Ray”, from Battlefield to Breakfast Cereal in a few short years.