RadioBanter - View Single Post

May 3rd 04, 05:17 PM

(Griff) wrote in message . com...
hi there everyone

I'm currently reading a book called "Code" by Charles Petzold which is
essentially a history and overview of computer technology, quite
entertainingly told. I've just read the section on telegraphs and
relays and am confused by something he has written.

He explains the basic principle of the telegraph, and goes on to show
with some simple Ohm's Law calculations that you can't extend the
length of telegraph wires indefinitely due to the loop resistance
etc., hence the need for the invention of the relay.

However, I am a bit puzzled by his summing-up quote for this chapter:
"Regardless of the thickness of the wires and the high levels of
voltage, telegraph wires simply couldn't be continued indefinitely".
Now I would have thought the restriction was *exactly* due to the
limitations on wire thickness and voltage (ie the practical problems
with manufacturing telegraph cables and voltaic cells at that time).
Or am I doing Mr.Petzold an injustice, when perhaps there was some
other limiting factor ?
I have read somewhere that there were problems with insulation for
very long cables, is this perhaps something to do with it ?

It's simply a matter of a poorly-written sentence. It should read
something like 'could not be economically or practically continued
indefinitely'.

*In theory*, a basic telegraph circuit could be any conceivable length
*if* there is no limit placed on the size of the wire used nor the
battery voltage. But in the real world, there were very definite
practical limits to how thick a wire could be used, and how high a
battery voltage could be used. And the economics of the hardware used
were a major consideration.

The early transatlantic telegraph cables were thousands of miles long
and used *no* repeaters or relays along their route.

For a more detailed historical account of the telegraph and its
development, I suggest the book "The Victorian Internet"

73 de Jim, N2EY