Now I have finished the 2nd experiment with silicon transistors – but after “power on” the bulb is ON, so I always have to STOP with the lower switch. Then all is working good, until the next power OFF / ON…

    Removing the resistor 1.5kΩ will cause malfunction, because the NPN transistor base is “hanging in the air” and cannot work as designed.

    The circuit only works rather well with Silicon transistors; I tried to use a PNP Germanium transistor (like in experiment #1) but this time without success. The block covers are equal, but everybody has to have a look inside: TO-1 outline = Germanium, TO-92 (or TO-106 in this photo) = Silicon.

    By the way: the transparent “Steck” blocks (plug-in) had the article numbers #8199002 / 8199003 (PNP), the “normal” versions #814405…8 (PNP/NPN).

    The battery block #8100540 (9V, neutral contacts – later #2124) was available since 1975 and came with the “Physik Experimentell” training set from Mr. Saucke (who wrote the Buchlabor and developed the “Spulenbaustein” #810505 (#2706) etc. The thyristor #8100423 (#2418) is – together with many other semiconductors – from the 197x years, so these all are “legacy” versions, too… May be a co-working of Mr. Guerth / Mr. Birett and Mr. Timmer around 1970.