Wednesday, 9 January 2013

Fish traps on shore leave

An interesting blog post of Christopher Hogue at BioImplement suggested that part of a spring loaded fish hook (patented by Job Johnson in 1847) took shore leave and became an integral part of a rat trap. The patented fish hook looks like this:
Figures from Patent US 5,256 (Johnson 1847)

The rat trap, however, looks like this:
Job Johnson's rat trap (linked from BioImplement)

Now, what do we have here?
A flat spring loaded rat trap with a bait hook reminiscent of a fish hook and a claim to embody US patent 5,256. Nevertheless, I have some caveats with the simple story that the patented fish hook has become integral part of the rat trap and, therefore, the latter carries the patent ad justly.

1.) The fish hook has a tension spring (contractile spring, d, in Johnson's usage) pulling the striker (dart b), whereas the rat trap has a torsion spring pushing the striker. 2.) The torsion spring runs around the pivot of the striker, while the tension spring runs around the handle (stock or frame a) of the fish hook, which is rectangular to the pivot. 3.) The bait hook of the rat trap runs through the wooden base and re-emerges at the top, where it holds the striker directly. That is, one part carries the function of bait hook at one end and of holding bar at the other. If the rat pulls the bait, the hook will be pulled down through the wooden base and thus release the striker at the other end. The set/release mechanism of the spring loaded fish hook is more complicated and indirect. There is an additional part between the hook and the striker -- the trigger-lever (see e in fig. 3 above) links arms with the cock-pin, 8, of the dart/striker. When the hook is pulled, the trigger-lever slips from the cock-pin thus releasing the dart.

Let's compare this with what Job Johnson claimed in his patent (letter in italics relate to parts given in the patent figures above):
"I do not claim to have invented the common fish-hook, as that is well known and in general use; but I do claim as new and of my own invention and desire to secure by Letters Patent of the United States--        1. The original application of the stock or frame piece a, the original application of the helical contractile spring d, together with the original constructive arrangement of these parts for the purpose, conjointly with a crooked and barbed dart, b, acting, through the cock-pin 8, trigger-lever e, and contractile helical spring d, to strike the fish or animal by disengaging the dart b from the trigger d [sic, must be e] through the combined action of the changeable hook f with and upon the foregoing parts, the whole constructively arranged and combined to strike the fish or animal with the dart b at the instant of the fish or animal biting at or touching the bait on the hook f, the whole effected without any action of the line or of the person holding the line, substantially as described and shown." (Johnson 1847, US patent 5,256)
Now that is some sentence. The last mention about no action on the line being necessary probably relates to fish hooks like the one patented by Stanton Pendelton the same day, 21 August 1847, as US patent no. 5,255. In it, the spring is missing and the power driving the striker down upon the fish is its own pulling at the line. The second claim of Johnson, which I did not quote above, relates to part 9 in the figures, which is a guard-ring protecting users from injury when setting the trap.

Basically, the long sentence quoted above claims that the whole arrangement including hook, tension spring, trigger-lever and striker is the patent. This patented mechanism, however, is different from the one operating the rat trap. The latter has no trigger lever and works with a torsion spring instead.

Strikingly, Johnson always mentioned "fish or animals" in his patent. It seems as if he did already have other ideas about its use. There is independent evidence in the form of a review of the patented fish hook in The Prairie Farmer concluding: "Those who wish to catch rats have got the right machine here." Todd Larson also wrote a book about fish hook history in which he details how farmers have been in the habit of using Johnson's fish hook for catching rats. (See BioImplement for source of quote and Larson's book.)

Maybe we can assume that Johnson had the idea of modifying his fish hook for catching rats and put the patent advert on the later rat trap in order to signal that it was of the proven quality the farmers already knew from his fish hook. The difference, however, was too large to claim embodiment of patent 5,256 in the rat trap.

In conclusion, the Johnson rat trap shows that traps for mice, rats and fish belong into one cluster of artifacts (traps) with rampant exchange (not of physical parts but of ideas) amongst them.

P.S.: I've just stumbled across another patent of a fish trap reminiscent, at first sight, of some mouse and bird traps (see here). It is often referred to as the Gabriel fish trap on the Internet:
this picture appears as a link to

The inventor's correct name was William Gabrielson, however, and the US patent no. 820,640 was of 1906. Here's a similar looking trap used for birds and mice.
"Cyprus" of Orlando Legget from British mouse traps..., p. 21 (PDF at

Although it looks as if the jaws of Gabrielson's fish trap turn on an axis rectangular to the handle, as in the Cyprus, they actually turn around an axis along the handle. This gets clear from the following patent drawing showing the trap half way between open and closed position. 
The superficial similarity turns out to contain a lot of difference at a closer look. Likewise, Johnson's rat trap is not really an embodiment of his patented fish hook despite his advertising it as such.