Static Characteristics in Common Emitter Mode

What this figure shows. An NPN transistor is biased with $V_{BB}$ (through rheostat $R_1$) providing the base-emitter voltage $V_{BE}$ and a microammeter reading base current $I_B$, and $V_{CC}$ (through rheostat $R_2$) providing the collector-emitter voltage $V_{CE}$ and a milliammeter reading collector current $I_C$, with the emitter common to both loops (confirming the common-emitter configuration) and carrying the combined emitter current $I_E$ -- this single circuit is the source of all three characteristic curves (input, output, transfer) described in the following sub-sections, simply by choosing which quantity is held fixed and which two are plo …