Hi, this book begins in 1963 with me as a child of 9 trying to understand why my crystal set worked OK with the GPO headphones but not with my home-made earpiece (or with a loudspeaker) - output impedance - that's why But it was not until I was 13 or 14 that I realised this because I did not believe in "output impedance" - how can something such as a battery have output resistance? It is supposed to be supplying current, not hindering it This mystery...