The handheld meter has been a staple in almost every technician’s toolkit for nearly 100 years. Today, the majority of handheld meters have digital displays. However, until the 1970s, most meters featured analog readouts. These were typically multimeters capable of measuring voltage, current, resistance, and occasionally capacitance and frequency.
“Pocket watch”-style measuring instruments were popular during the 1920s and 1930s. They were mainly used to check battery voltages in early battery-powered radios and telephones, and in cars to test the accumulator (battery) voltage. It is not known exactly how many were manufactured, but it’s likely that every electrical technician and car enthusiast owned one—just as every engineer had a slide rule, and every doctor had a stethoscope.
At the time, a typical unit could be purchased for $1–2 (equivalent to $15–30 today). Popular brands included Sterling, Readrite, Eveready, and Hickok. Today, good examples can often be found for $5–8 at swap meets and flea markets—though unfortunately, the original test leads are often missing. You can find many of these meters at: https://www.brundrit.co.uk/pocket-voltmeters.
Zwar’s Pocket Voltmeter
This elegant pocket-watch-style meter is considered a rarity. It was manufactured in 1930 in Budapest by Hungarian electrical engineer Ferenc Zwar. This small moving-coil (Deprez or D’Arsonval) galvanometer was primarily used to measure voltages in radio battery circuits. It can measure both voltage and current, with ranges of 3V, 15V, 150V, and 30mA, 300mA.
The instrument features a nickel-plated case measuring 59 mm in diameter and 30 mm thick. The basic 3V range can be increased using external ballast resistors or, when measuring current, by flipping a slide switch on the side of the case.
To access the internal parts, you must carefully remove the back or front cover using a watchmaker’s case knife. Hold the meter by the body, insert the blade into the slot under the cover, push it all the way in, and gently twist to pop the cover off.
Inside, the moving coil is extremely delicate—thin wire stops require careful handling. As its name suggests, the moving coil meter operates using a lightweight coil suspended in a magnetic field provided by a fixed ring-shaped magnet. When current flows through the coil, it creates a magnetic field that interacts with the permanent magnet, causing the coil (and the attached needle) to move. Two hair springs control this deflection, ensuring the movement is proportional to the current. The meter measures only direct current (DC). Its internal resistance is 250 ohms, and it requires 750 µA for full-scale deflection.
Circuit Explanation
To expand the current range of the basic meter, a 6.5-ohm resistor (R2) is placed in parallel with the movement. This shunt resistor allows the total current range to increase to 30 mA.
Due to the movement’s sensitivity, it can measure only small voltages (0.1875 V). The R4 resistor increases the total resistance to 1000 ohms, enabling a 3 V voltage range. This low input resistance is acceptable for battery measurements but not ideal by today’s requirements.
To further extend the current range to 300 mA, you can close the S1 switch, which activates an additional shunt resistor (R3). In this configuration, the total resistance becomes 10 ohms, and the voltage drop across the instrument is 0.01 V/mA.
For higher voltage ranges, external resistors must be used. A small round black box was supplied with the meter, containing wire-wound resistors on a spool. It features three test points: 3 V, 15 V, 150 V. The test lead connects directly to the 3 V pin. Between the 3 V and 15 V pins is a 400-ohm resistor, and between 15 V and 150 V, a 4500-ohm resistor, both in series as is typical.
Accuracy and Build
Despite its handmade construction, the instrument delivers remarkably good accuracy:
Unlike today’s ergonomically designed meters with rubberized plastic cases, early pocket meters were built with metal enclosures and wooden boxes. While the metal case is insulated from the circuitry, it is not safe against electrical shock. Remember—these meters were intended for low-voltage battery testing.
Historical Context
Between the two world wars, Budapest was home to numerous electrical companies, such as:
- Thermolux Electricity Company
- Béla Szalkay High Voltage Electrical Appliances Factory
- Gyula Gárdos Electrotechnical Factory
- Magyar Brown Boveri Művek
- Zwar Ferenc Precision Electrical Instrument Laboratory
- …and many more
Unfortunately, very little information is available about the creator of my small instrument, Ferenc Zwar. His name appears in technical journals and company directories from the time, but not frequently. In his advertisements, he offered his instruments to radio technicians, electrical engineers, and apprentice instrument makers. He operated his own electrotechnical and scientific instrument laboratory between the wars at 3 Héderváry Street (now Benyovszky Móric Street, in the 8th district of Budapest). The building was destroyed during World War II.
As for his family, I found that Ferenc Zwar married Klara Zimmermann, and their daughter, Anna Zwar, married a Hungarian army officer, Lajos Gidófalvy, in 1933. In 1945, Gidófalvy died a heroic death while trying to prevent the Nazi demolition of the Elisabeth Bridge. He was posthumously promoted to major (in 1948) and then colonel (in 1991). One of the streets in the new officers’ housing district, built in the 1970s, was named after him.
Who Owned Tesla’s Voltmeter?
Tesla’s voltmeter was probably not made by Zwar, and it is not even certain that it is of Hungarian manufacture. There are no maker’s marks on it. It has two test points and two voltage ranges: 0–12 V and 0–120 V. Its scale is non-linear, which might suggest that it is not a Deprez-type instrument but a moving-iron type capable of measuring alternating current as well. It bears the inscription: “Nicola Tesla, 1881.”
Thinking of AC, it is tempting to believe that the instrument once belonged to Nikola Tesla. Tesla lived in Budapest only briefly, in 1881, which matches the year engraved into the cover. The problem is that the moving-iron instrument was invented by the Austrian engineer Friedrich Drexler in 1884, by which time Tesla had already emigrated to the United States.
However, a closer examination reveals that this instrument is in fact a Deprez-type meter. The polarity markings (+) beside the probes confirm that it was designed for direct current only, and the non-linear nature of the scale is simply the result of its internal design—probably an overly short spiral spring—rather than evidence of a different operating principle. This arrangement of a coil moving in a narrow air gap with a spiral spring was developed in 1882 by the French physicist Jacques-Arsène d’Arsonval and the French engineer Marcel Deprez. Therefore, in 1881 Tesla could not have had such an instrument in Budapest.
This particular meter was probably manufactured sometime around 1925–1930. Despite appearances, it is not a moving-iron instrument and cannot measure alternating current. The non-linear scale is likely due to the quality of the hairspring. In Hungary, the Engel Károly Factory for Electrical Fittings and Apparatus (EKA) produced such instruments until the 1950s, after nationalisation too. The design and materials of the “Tesla voltmeter” are very similar to instruments made by EKA in the 1930s, but it lacks the characteristic EKA logo. It is also possible that it was made in small series by the Fehér Ferenc Electrical Precision Instrument Factory (64 Üllői Street), or perhaps it is the product of a foreign, possibly German, workshop. We simply do not know.
Although it certainly never belonged to Nikola Tesla, it remains beautiful and valuable memento.
Comparison of Pocket Voltmeters
| Feature | Zwar Pocket Voltmeter | “Tesla”-Engraved Pocket Voltmeter |
|---|---|---|
| Year of Manufacture | 1930 | Unknown (engraved date: 1881, probably made in the 1920s or later) |
| Manufacturer | Ferenc Zwar, Budapest | Unknown (possibly EKA or German workshop) |
| Instrument Type | Moving-coil (D’Arsonval) | Moving-coil (D’Arsonval) |
| Polarity Sensitivity | Yes (DC only, direction matters) | Yes (DC only, direction matters) |
| Scale Type | Linear | Nonlinear |
| Voltage Ranges | 0–3 V, 0–15 V, 0–150 V (with external resistors) | 0–12 V and 0–120 V |
| Current Ranges | 0–30 mA, 0–300 mA (with shunt switch) | None (voltage measurement only) |
| Case Material | Nickel-plated metal | Metal, pocket-watch style |
| Size | 59 mm diameter, 30 mm thick | Pocket-watch size |
| Markings / Labels | ZWAR F. Budapest | No external markings |
| Probable Origin | Hungarian (Zwar’s workshop) | Probably Hungarian (EKA) or German origin |
| Current Location | Private collection | Technical Study Collection, Budapest |