Aluminum rectifiers (was Dummy Loads)

Yes... Old books on technical subjects are similarly interesting. You find all sorts of things in them that them pop up again later. For example, I have a set of books called 'Modern Electrical Engineering' which appear to date from just after the First World War. I bought them becuase when I flipped through them in a second-hand bookshop, I saw a good description of the Baudot telegraph. But after readingg them mroe fully I found somethign that was even more interesting. A description of the signaling system used on the London Underground ('Tube', call it the Subway if you must :-)). Apparently even then there were too many trains for the signalmen to remember the order. They had an electormechanical FIFO buffer. A drum with 4 rows of pins round it. Each axial positon of 4 pins on the drum corresponded to a word of the store and held the description of one of the trains on the line. A 'write device' moved the 4 pin in the current postion in or out, encoding the description of a particular train. A read device sensed the psitions of the pins. The read and write devices moved independantly relative to the drum, which makes the whole thing very similar to the stnadard implementation of a FIFO buffer using read and write pointers. When a new train came into that block of track, its description was stored as a 4 bit code in the next position on the drum and the write device moved on to the next (unused) position. The read device effectiviely told the signalman what the next train to come out of that block would be, when it emerged, the read device was moved on one position so as to look at the next train's word, and so on. Oh yes, the 4 bits read from the store were decoded to 1-of-16 using a relay tree and then used to operate indicator lights for the signalman. There is one of these devices as a static exhibit in the London Transport Museum, but when I visited it (some years ago, I admit), the description wasn't too clear as to how it worked. Having read the book, I realised what it was. There are, of course many other examples -tony