Conductivity of suspensions of cream in its skimmed milk.—The well known formula for the conductivity of a suspension of non-conducting spheres is $\frac{(k_1-k)}{(2\mathrm{}{k}_1+k)}=\frac{1}{2}\rho$, where $k$ and $k_1$ are the specific conductivities of the suspension and of the suspending medium and $\rho$ is the volume concentration of the disperse phase. This formula was experimentally verified within about 0.5 per cent for values of $\rho$ up to 62 per cent, by comparing the values of $\rho$ given by the formula with the values obtained directly from the known dilutions. Since the conductivity was found to change with the age, due to incipient souring, measurements were made quickly. Stirring was used instead of shaking to avoid introducing finely divided air, and was effected by using a special cell in which a bead rolled back and forth as the cell was rocked. After stirring was stopped there was a rapid increase of resistance due, perhaps, to the particles taking up an ordered arrangement. The temperature coefficient was found to be the same for the cream as for the skimmed milk, decreasing from 2.65 per cent at 6° to 2.12 per cent at 25°C. The variation with frequency was a 2 per cent decrease of conductivity as frequency was increased from 1600 to 200,000. These results indicate that the measurement of conductivity furnishes a practical method of determining the butter fat in milk and cream, which is as accurate as those now used.

• Received 8 October 1924

DOI:https://doi.org/10.1103/PhysRev.25.361