According to the formulae the pressure drop R (l) and the degree of ventilation V (l) of cigarettes depend on the length l of the cigarette without tip, on the pressure drop RG of the incandescent zone alone, on the specific resistance to air flow b of the tobacco rod, on the specific conductivity a of the paper wrapping, and on the pressure drop RM of the ventilation-free cigarette tip. The parameters a and b are defined by the equations respectively, for the pressure drops RT and RP of the tobacco rod only and of the paper wrapping alone. In practice, the cigarette can approximately be considered as "ohmic" resistance to air flow, i.e. there is a linear relationship between pressure difference and flow rate. The degree of ventilation is the part of the total stream which penetrates the paper wrapping of the cigarette. The indicated relationships were theoretically deduced and empirically confirmed. The formula for R (l) corresponds to In general, there are the following relations where U1 is the total decrease of pressure occurring along both cigarette without tip and incandescent zone, I1 is the total stream leaving the cigarette at the cigarette tip, U2 is the decrease of pressure occurring along the incandescent zone, and I2 is the air stream which flows into the cigarette through the incandescent zone. The connections between U1, I1, U2, I2 and R, RG, V are as follows:
