For over 40 years Utah State University/Space Dynamics Laboratory and their predecessors have built instruments to measure electron density in the ionosphere. These instruments have incrementally improved as new measuring techniques have developed and new technologies have been introduced. This thesis introduces the properties and measurement of electron density in the ionosphere. It maps the instrument advancements from the early radio frequency probes of the 1960's to the present Plasma Impedance Probe (PIP), with emphasis on the introduction of the phase lock looped Plasma Frequency Probe (PFP) and the rediscovery of the Swept Impedance Probe (SIP). The current USU instrument's operation will be detailed. The calibration of the PFP and SIP will be presented, including development of new PIP calibrators. The PFP's loop characteristics and optimization are discussed. Lastly, the current status of the PIP electron density measurement is given, as well as recommended improvements.