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.