We report on the first observations of radio emission from transverse currents induced in a secondary cascade by a magnetic field in dense media. Only the Askaryan effect has been observed in previous experiments in ice, silica sand and in rock salt. These measurements are important because the radio detection of ultra-high energy cosmic rays (UHECRs) relies on models of radio emission from extensive air showers. In order to validate the models we performed an experiment at SLAC National Accelerator Laboratory in 2014 using a high density polyethylene (HDPE) target placed in a magnetic field, yielding results within 36% of the model predictions. Show Partial Abstract