= Coulter counter
{c}
A <Coulter counter> measures suspended particles by detecting electrical pulses as they cross a constricted conducting path. A particle with different electrical properties from the suspending fluid changes the <electric current>. Counting pulses in a measured <volume> gives an estimate of particle abundance, with <dilution> used to recover the value for the original sample.<Means for counting particles suspended in a fluid by Wallace H. Coulter>[US2656508A, description of Figs.1–4]{ref}
<Wallace H. Coulter>'s patent, filed in 1949 and granted in 1953, describes two insulating vessels, electrodes and a narrow passage, with detection circuitry registering particle events. It also identifies masking when particles occupy the sensitive region together.<Means for counting particles suspended in a fluid by Wallace H. Coulter>[Publication history; Figs.6–7 and masking discussion]{ref}
For a synthetic example, 2,500 accepted pulses in $50\,\mu\mathrm L$ of a 100-fold diluted suspension imply
$$
c_{\mathrm{original}}=\frac{2500\times100}{50}=5000\ \text{particles per microlitre}.
$$
The estimate assumes representative sampling and correctly counted events. A pulse is not automatically a <blood cell> of a particular type: debris, coincident passage and classification thresholds affect what is counted.
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