What is the side slot on the HP 41CX for?

[Jim Horn]

Actually, the homebrew cable using a modified TV power cord and an external 6 volt lantern cell was popular among some of the hard core '41 users due to the terrible effect the card reader had on the expensive N cell alkaline batteries. Those batteries cost about US$1.50 each back then (1979 - or over US$6 in today's money) but the over 1 ampere load placed by the card reader motor caused them to fail in card reader use once 10% of their capacity had been used (i.e. they still had 90% of their charge left). For anyone doing heavy '41 coding with magnetic card use (such as at club meetings making copies of a popular program for others), this was a real benefit.

Of course, the use of NiCd cells and later, HP's own NiCd battery pack, reduced the need for such kludges. But such is the way of progres...