You could call Cap a drum enthusiast after reading his most interesting letter on making a drum. By the same token you could call me a "travelling drum enthusiast": One can obtain the necessary displacement for successive lines either by causing the penmotor to move in a path parallel to the drum axis or by making the drum travel axially in front of a stationary penmotor. (See Martin G Murray's 1938 design on PSN recently). The second version is far simpler to construct and gives one much freedom to experiment with different penmotors. I still have the little drum assembly which I made for a drum chronograph in 1957 during those heady Sputnik/Moonwatch days. As for constructing a drum from scratch, there is no law which states that one's chart speed must be 30 or 60 or whatever mm/minute. It is far simpler to find a thin walled round tube tube of metal, plastic or even cardboard and fit two end pieces. Then one simply makes a scale for measuring the time interval from the nearest time marker so as to fit in with the chart speed, which is fixed by whatever drum diameter you have and the motor speed. Amateur constructors need not be daunted at the prospect of having to buy expensive penmotors. An adequate substitute can be made, based on a loud speaker moving coil assembly. The cone can provide the necessary restoring force or if one is adventurous one can dispense with the cone and use an external spring to provide the restoring force. The somewhat limited movement of the coil needs to be amplified and of course transmitted to the writing stylus. Model airplane balsa is good for the linkage especially if one wants a penmotor with a fast response. Blobs of silicone rubber judicially applied can serve as friction-and shake free bearings. The experimenter only needs these simple freely available materials and a little bit of imagination. One also needs to protect these home made devices from domestic accidents such as the family cat "calibrating" the equipment in hot pursuit of a mouse. Cheers all Danie Overbeek. __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>