Magnetic Drum

Part of Data Storage

The first practical random-access bulk storage device — a rotating cylinder with fixed read/write heads that enabled real-time computing.

Why This Matters

The magnetic drum was the bridge between sequential storage (tape, punched cards) and true random-access storage (hard disk). Invented in 1932 by Gustav Tauschek and deployed in major computers from the late 1940s onward, the drum provided something transformative: the ability to access any stored word within one rotation period (a fraction of a second) rather than waiting for a tape to wind forward.

For rebuilders, the drum has an important advantage over the hard disk: it is mechanically simpler. Hard disks require multiple precision-lapped platters spinning at high speed with read/write heads floating micrometers above the surface on an air cushion. Drums use fixed heads that maintain a fixed gap from the surface — easier to manufacture, more tolerant of vibration, and easier to diagnose when they fail.

A working magnetic drum can be built from salvaged motor components, a machined aluminium or steel cylinder, oxide coating, and modest electronics. It was done in university laboratories and early computer companies in the 1950s with far less manufacturing infrastructure than exists today.

Physical Construction

A magnetic drum consists of a cylindrical surface coated with magnetic oxide (the same materials used in tape and disk recording), rotating on a horizontal axis at constant speed. Fixed read/write heads are positioned at precise radial distances from the surface, with a gap of 0.025–0.25 mm between head face and drum surface.

The drum cylinder: Historically made from aluminium alloy for lightness and dimensional stability, or steel for rigidity. Typical dimensions ranged from 20 cm to 50 cm diameter and 30 cm to 100 cm length. The surface must be machined to high roundness (less than 0.025 mm runout) so that the head gap remains consistent through each revolution. For a scratch-built drum, a lathe is essential; the final surface finishing requires grinding or careful honing.

The magnetic coating: Applied identically to tape production — ferric oxide particles in a polyurethane binder, sprayed or brushed onto the drum surface and cured. After curing, the surface is lapped smooth. Achieving uniform coating thickness is the main challenge; imperfections cause signal level variations that the read electronics must accommodate.

Head positioning: Unlike hard disks where heads fly on an air bearing, drum heads are fixed and maintain a mechanical gap. The head body (ferrite core) is mounted on a precision bracket with adjustment screws to set the gap. A gap of 0.05–0.15 mm is practical for a scratch-built system; smaller gaps enable higher recording density but are harder to set and maintain.

Rotation speed: Early drums spun at 1,000–12,500 RPM. At 3,000 RPM, one revolution takes 20 milliseconds — the maximum latency to wait for a specific position to pass the head. Higher speeds reduce average latency but require more precise balance and better bearings.

Tracks and heads: Each head covers one circular track around the drum’s circumference. Multiple heads are mounted at different axial positions along the drum, each covering a different track. A drum with 64 heads covers 64 tracks simultaneously; data from any track is available within one revolution with no head seeking required. This is the drum’s key advantage over early disk drives, which had a single moveable head that had to seek to the right track.

Addressing and Data Organization

Data on a drum is organized into sectors or words within each track, and addressed by track number (which head) and sector number (angular position).

Sector timing: A timing track (one dedicated track with a fixed pattern) provides sector markers. A photocell or Hall-effect sensor detects a reference mark on the drum once per revolution, providing absolute position. The controller counts timing pulses from the timing track to determine when a specific sector is under the head.

Access time: The time to read a specific word consists of:

• Rotational latency: 0 to one full revolution (average: half a revolution). At 3,000 RPM, average latency is 10 milliseconds; worst case is 20 milliseconds.
• There is no seek time for drums — heads are fixed per track. This distinguishes drums from hard disks, where the head must physically move to the correct track, adding 5–20 ms of seek time.

Word length and track capacity: Early drums stored one machine word (typically 32–50 bits) per sector, with sectors as small as 30 microseconds apart. A track on a drum spinning at 3,000 RPM with 100 sectors stores 100 words. With 64 tracks and 100 sectors per track, total capacity is 6,400 words — perhaps 25–50 kilobytes. Modest by modern standards, but adequate for many early applications.

Electronics and Interface

Each read/write head requires its own amplifier chain. Write electronics must generate a current pulse sufficient to saturate the recording medium. Read electronics must amplify the tiny voltage (microvolts to millivolts) induced as magnetized regions pass under the head gap.

Write circuit: A current source with polarity control, driven by a data flip-flop. When the flip-flop is 0, current flows in one direction through the head coil; when 1, current flows in the opposite direction, reversing the magnetization. Transition points (where the current reverses) correspond to recorded bit transitions in most recording codes.

Read circuit: A low-noise preamplifier (first stage) close to the head, followed by a filter to select the signal frequency band, followed by a comparator that converts the analog voltage to a digital 0 or 1. The transition detector converts peaks in the read signal to digital pulses representing bit transitions.

Drum controller: Coordinates sector timing (knows where the drum is in each rotation), manages read/write operations for each head, and presents a standardized interface to the CPU. The controller buffers one sector of data for each active head, transfers data to CPU memory via direct memory access or programmed I/O, and handles retries on read errors.

Building a Minimal Working Drum

A practical small drum for a post-collapse computing system:

Specifications: 20 cm diameter cylinder, 40 cm length, 4 tracks (4 heads), 3,000 RPM, 32 sectors per track, 8-bit words per sector. Total capacity: 4 × 32 × 8 = 1,024 bits = 128 bytes. Minimal, but sufficient to store and retrieve data tables or program constants.

Materials needed:

• Aluminium alloy cylinder, machined to 0.025 mm roundness
• 0.75 kW single-phase induction motor with speed regulation
• 4 ferrite ring cores, 5 mm OD, 0.1 mm gap, wound with 200 turns of 0.1 mm enamelled copper wire
• Oxide coating: 100 g iron oxide (from salvaged tape), 20 g polyurethane binder, solvent
• Precision brackets (machined steel) for head mounting
• One photocell + IR LED pair for revolution reference
• 4× LM387 preamplifier ICs for read channels
• 4× LM393 comparators for bit detection
• One small programmable logic device or discrete logic for sector timing and interface

Assembly sequence:

1. Balance the cylinder dynamically (add/remove material from machined recesses to achieve < 1 gram-centimeter imbalance at 3,000 RPM).
2. Apply oxide coating in two thin layers, curing each at 60°C for 4 hours. Sand smooth between coats.
3. Mount cylinder on motor shaft via a flexible coupling (allows slight misalignment without vibration transmission).
4. Install heads, adjust gap to 0.1 mm using feeler gauges, lock with Loctite.
5. Wire read/write electronics and test each head individually with an oscilloscope.
6. Calibrate sector timing against revolution reference pulse.
7. Test full read/write cycle with known data pattern.

Historical Context and Applications

The drum was the primary working storage device for early computers that could not afford (or had not yet developed) disk drives. Notable drum-based machines:

• ENIAC (1945): Used no drum, relying entirely on vacuum tube registers and punched cards. The drum came slightly later.
• ERA 1101 (1950): First commercial drum-based computer. 1 MHz drum, 16,384 words, each 24 bits. Used for US Navy code-breaking.
• IBM 650 (1954): Most widely sold computer of the 1950s. 2,000 word drum, 12,500 RPM, 2.4 millisecond average access time. Thousands of units sold to universities and corporations.
• Manchester Mark 1 (1949): Used a Williams Tube for main memory and drum for bulk storage.

By the early 1960s, moving-head disk drives began to displace drums for bulk storage, offering higher capacity per unit volume. But drums continued in use as fast main memory (before semiconductor RAM) until the early 1970s.

For a modern rebuilder, the drum represents a sweet spot: more complex than paper or tape, but far simpler than a precision disk drive, while offering genuinely random (or at least semi-random) access with latency measured in milliseconds rather than the seconds of tape rewinding.