text: John A. Simpson, at the University of Chicago, invented and developed the neutron monitor over the years 1948-50 and found that the Earths magnetic field could be used as a spectrometer to allow measurements of the cosmic ray spectrum down to low primary energies. The magnetic latitude of a particular neutron monitor determines the lowest magnetic rigidity of a primary that can reach the monitor, the so-called cut-off rigidity. The stations altitude determines the amount of absorbing atmosphere above the station and hence the amount of absorption of the secondary cosmic rays (the higher the station, the higher the counting rate). By using a combination of lead (to produce local interactions), paraffin or polyethylene (to moderate or slow down the neutron component) and multiple slow-neutron counters, Simpson greatly increased the counting rate in his monitor design.