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30 NOVEMBER 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 11 FEATURE ARTICLE FIGURE 5. Rhodes Potentiometer Voltmeter Model P. FIGURE 6. Weston DC Millivoltmeter Model 622. current flow was determined to be detri- mental, they installed drainage bonds between the pipe and the rail itself, or at railway substations, to drain excessive cur- rent from the affected structure. In its sim- plest form, a bond was a wire that con- nected a rail to a buried pipe. To monitor and manage this current exchange, amper- age and voltage measurements were taken at bond stations using ammeters, voltme- ters, and shunts. Ammeters measured the amount of electric current in amperes in a circuit. Voltmeters measured the electrical potential difference between two points in an electrical circuit. Shunts were used to measure current. When a shunt was placed in the wire or bond that connected the rail and the pipe, current passing through the drainage bond could be measured. By knowing the resistance of the shunt and measuring the voltage with a millivolt - meter, the amount of current passing through the shunt could be calculated using Ohm's Law (I=V/R, where I is current, V is voltage, and R is resistance). Ammeters in the EBMUD collection include the Weston Mil-Ammeter Model 264 (Figure 1), manufactured by the Weston Electrical Instrument Corp. and first pat- ented in 1888, and the General Electric Thomson Alternating Ammeter (Figure 2), first patented in 1895 and standardized at the Lynn Laboratory in 1905. According to General Electric, "This instrument is intended to be used in a horizontal posi- tion. It is practically correct for alternating currents of any frequency or wave form." A 200-A capacity Portable AC Ammeter (Figure 3), manufactured by the Western Electro-Mechanical Co., Inc., is also part of the collection, as well as a Weston DC Ammeter Model 45 (Figure 4), manufac- tured in 1948 by the Weston Electrical Instrument Corp. and used in conjunction with an external 50-mV shunt. This popular ammeter was extremely accurate for mea- surements between 0.01 and 20 A. The collection also includes several voltmeters. The 1941 Rhodes Potentio - meter Voltmeter Model P (Figure 5), manu- factured by the Sensitive Research Instru- ment Corp., was designed particularly for measuring the voltages encountered in electrolysis and was probably used in the field to measure stray current at rail bonds. This very versatile instrument could also measure electric potential. The original cost was $258. The Weston DC Millivolt - meter Model 622 (Figure 6), manufactured by the Weston Electromechanical Corp., features a Bake lite plastic case. This model was standardized in 1936. The Westing- house Electric 600-V capacity AC Voltmeter (Figure 7) also has a Bakelite case. Bakelite was the first synthetic thermoset plastic and was known for its electrical noncon- ductivity, although it was also used in cos- tume jewelry and kitchenware. Various shunts used by the Electrolysis Department that are part of the collection include a DC shunt (Figure 8) manufac- tured by the Weston Electrical Instrument Co. Constructed of a ventilated cherry wood box that houses a long, accordioned ribbon of resistance wire, this 150-A device likely predates the present-day shunt design patented in 1893. Other antique shunts include a 50-A shunt on an oak base (Figure 9) and a 150-A shunt on a wood base (Figure 10), both developed after 1893, that feature resistance ribbons made from an 86% copper, 12% manganese, and 2% nickel alloy known as Manganin; a 10-A shunt with a Bakelite base and a delicate resistance element that is covered with a non-metallic shield (Figure 11); a 10-A shunt with an oak base and oak-covered resistance wire (Figure 12); and a 600-A shunt (Figure 13) that was used in electrical bonds between water mains and the elec- tric street car rails. Some shunts were designed to be mounted permanently and some were portable. Shunts such as these are still commonly used in CP rectifiers and anode junction boxes. The shunts in this collection were primarily used in trolley drainage bonds. Another piece of equipment used in conjunction with drainage bonds was the recording voltmeter. A pair of wires led from the shunt to the recorders so voltage could be measured continuously. The early units were called smoked-chart recorders. These devices used rotating circular chart paper that had been blackened with smoke residue. As the disc rotated, a stylus would scratch off the residue, which left a trace that corresponded to the voltage being measured vs. time. In San Francisco's East Bay, numerous smoked-chart recorders were assembled in one central location to monitor bonds throughout the area