The term ``glo‐ball'' is used to designate a small, thin‐walled, partially evacuated glass sphere containing helium, used for investigating electric field distributions in rf resonant cavities. Ionization, or firing, of the gas in the glo‐ball indicates a given potential gradient across it; in field investigations a monitoring loop is used to record the voltage required in the cavity to produce this ionization. Glo‐balls were developed specifically for their application to linear accelerators.Experiments were conducted to provide a method of making glo‐balls, under controlled conditions with known constituents, that would have a stable and relatively low firing gradient.Factors associated with the making of a glo‐ball were determined, and a spectroscopic analysis identified the gas components. It is now possible to make a ball that fires at a minimum voltage gradient with excellent stability. Other characteristics of the glo‐ball pertinent to its use as a field‐measuring device were investigated.