Abstract: A battery charger of the type which provides a constant current charge for charging batteries which may be used to charge batteries having a wide range of voltages and capacities. The charging circuit of the constant current battery charger of the present invention includes a conventional voltage source, three terminal regulators, fast and float charging rate resistors, a resistive divider for sensing battery voltage and integrated circuit means for providing drive current for a switching relay and for providing hysteresis. The circuit provides ready and complete adjustability of voltage range, fast and float current ranges and switching hysteresis range wherein these ranges are defined by linear relationships and may be changed by substituting passive components such as resistors.

Abstract: A mine roof bolt and a method of measuring the strain in mine roof bolts of this type are disclosed. According to the method, a flat portion on the head of the mine roof bolt is first machined. Next, a hole is drilled radially through the bolt at a predetermined distance from the bolt head. After installation of the mine roof bolt and loading, the strain of the mine roof bolt is measured by generating an ultrasonic pulse at the flat portion. The time of travel of the ultrasonic pulse reflected from the hole is measured. This time of travel is a function of the distance from the flat portion to the hole and increases as the bolt is loaded. Consequently, the time measurement is correlated to the strain in the bolt. Compensation for various factors affecting the travel time are also provided.