Simple and reliable relaxation-oscillation (RO)-driven dc SQUIDs (superconducting quantum interference devices) have been fabricated with a flux-to-voltage transfer factor H up to 1 mV/microH0 and internal energy sensitivity Ev of (4 +/- 1) x 10-31 J/Hz at frequencies above 10 Hz and 1 x 10-29 (1 Hz/f) J/Hz at lower frequencies. Supplied with a 50-micron-wire 20-turn spiral input coil and a 6-sq cm pickup loop, such SQUIDs provided magnetic field sensitivity close to (6 +/- 3) x 10-15 T/Hz exp 1/2 above 10 Hz. The dynamic range of the magnetometers was close to 1.5 x 107 Hz exp 1/2 above 10 Hz. The dynamic range of the magnetometers was close to 1.5 x 107 Hz exp 1/2, and the maximum slew rate was about 10 microT/s at 0.1 kHz. Successful records of earth-ionosphere magnetic resonances have been obtained with these magnetometers housed in a stainless steel transport dewar. A balanced version of the RO-driven dc SQUID is suggested with improved H and Ev.