These video's are made for beginners in electronics and all people interested in these matters. They show how to create some basic-circuits. Many electronic fenomena are left out. Here you see how to create a variating voltage to drive an electronic (detection) circuit or detect current when a transistor or tube is used as a current amplifier. There is one omission in the tube-circuit: you need a resistor from kathode to minus (earth) to really get a negative grid voltage, I mean more negative then ground level (0), sorry. This matter is on two other videos on my channel. Ohms law (U=IXR, voltage = amps x resistance) can be applied on the drawn circuits. Tags: variable voltage, voltage drop, voltage divider, amplification, triode, negative grid voltage, grounded emitter, frequency dependant amplification, bipolar transistor, bias voltage. Praktical application: temperature meter, light meter, audio amplifier, HF amplifier My books: "Schematics 1" (www.lulu.com/content/4734386) and "Schematics 2, Audio amplifiers and loudspeaker boxes" (www.lulu.com/content/7658905)