Final answer:
A voltage tester or multimeter designed for high voltage systems is the tool used to test the 3rd rail. These devices ensure the integrity of the power supply by measuring terminal voltage and internal resistance while safely dissipating the power during the testing process.
Step-by-step explanation:
The hand-held voltage-testing device used to test the 3rd rail is a type of voltage tester or multimeter, which is specifically designed to handle the high voltages typically found in the power supply systems of railways, particularly those that use a third rail for electric power distribution. These devices may resemble the larger battery tester used by a U.S. Navy electronics technician as shown in Figure 21.15 or may have features similar to the smaller device with a digital display used for small batteries. These testers measure terminal voltage under load to assess the condition of the power source, in this case, the third rail's power integrity. Moreover, these tools can test for internal resistance, which is indicative of the health of the electrical system. When dealing with high voltage systems such as third rails, it's crucial that the device can manage and safely dissipate the power to prevent overheating or damage while conducting tests to verify that there is zero voltage before maintenance is performed, as noted in section 26.