Starting at the top and working down, the facilities in the supply space are:

• Static wire: a grounded wire at the very top of the pole intended to protect lower conductors from lightning.
• Transmission (sometimes called sub-transmission to distinguish it from transmission lines supported by steel structures): three uninsulated conductors which carry 3-phase high voltage (typically 69 to 200 kilovolts) circuits among substations.   This circuit may be wye-connected or delta-connected; if wye-connected, a fourth conductor, called the MGN, is required...
• MGN (multi-grounded neutral): a single uninsulated grounded conductor.   The currents in the three phases of the transmission (or sub-transmission) line are never quite equal; if they are wye-connected, the MGN carries the residual unbalance current.   At many poles, the MGN is physically grounded to a groundrod at the base of the pole.
• Primary Distribution: one to four uninsulated conductors, frequently supported on a crossarm, which carry power from substations to pole-mounted stepdown transformers.   The primary circuit (so named because it feeds the primary winding of the stepdown transformer) may be single-phase or three-phase, and typically operates at 4 to 15 kilovolts.
• Secondary Distribution: one or more insulated conductors, accompanied by an uninsulated grounded neutral conductor (which may be the MGN).   The secondary circuit (so named because it is fed from the secondary winding of the stepdown transformer) provides the standard 3-wire single-phase 115/230-volt electric service for residential and small commercial customers.   On special order, three-phase service and/or higher voltages can be provided for specific customers.   Secondary distribution conductors are usually twisted together in a bundle called duplex (two conductors), triplex (three conductors), or quadruplex (four conductors), although older secondary distribution circuits may consist of separate open wire conductors.
• Stepdown transformer: an oil-cooled transformer which converts the primary distribution voltage to the secondary distribution voltage.   Most stepdown transformers are designed for single-phase operation; if a three-phase secondary circuit is required, three physical transformers are usually required, and may be mounted the same pole as shown in the photo above.

This space is specified by the National Electrical Safety Code to separate electric supply conductors from communications conductors.   Under this code, the highest cable in the communications space must be separated from the lowest electric-power conductor in the supply space by specified distances.   The separations shown in this diagram are typical, although there are numerous exceptions and special cases.

The safety zone space protects communications personnel from dangerous voltages (even if a technician "forgets" to wear a hardhat).

Typical communications cables include:

• Telephone: telephone cables supported by steel strand.   Each telephone cable contains several individual copper wire pairs; a large cable may contain as many as several hundred pair.   The strand is placed under tension to prevent excessive sag; typical strand tension is a few hundred pounds, although a strand supporting a large multipair cable may be tensioned as high as 1000 pounds.
• Cable TV: CATV coaxial cable and equipment supported by steel strand.   An expansion loop at each pole absorbs expansion and contraction caused by temperature variations.   The strand is placed under tension to prevent excessive sag; the typical strand tension is a few hundred pounds.
• Other: just about any other type of communications circuits.   Among the more common are fire- and police-alarm wiring, traffic-signal control wiring, and closed-circuit audio or video communications circuits.   Depending on purpose and age, these circuits may utilize open-wire conductors, twisted-pair cables (similar to telephone networks), coaxial cables (similar to CATV networks), or fiber optic cables.

• Primary distribution.
• Secondary distribution.
• CATV cables.
• Telephone cables.
• Traffic signals.
• Traffic signal wiring.

• There is no static wire.
• The MGN is on the same crossarm as the primary conductors.
• The pole includes an unmetered streetlight, wired directly to the secondary.
• The CATV network is offset on a sidearm probably to maintain clearances from other facilities.

• A static wire.
• A large transmission line, probably at least 69-Kv.
• Secondary distribution wiring (both open-wire and triplex).
• A streetlight, hardwired directly to secondary distribution.
• CATV cables.
• Two telephone cables, with strand-mounted splice cases.
• A pole-mounted telephone cross-connect panel.

• Two primary distribution circuits.
• One open-wire secondary distribution circuit.
• Traffic signal wiring.
• Fire alarm wiring.
• CATV network.
• Seasonal decoration.
• Traffic signals
• Pedestrian walk signals.

Note that the fire-alarm wiring is open wire, and that it appears to be very close to the CATV cable.   If these wires are not insulated, this would constitute a violation of the National Electrical Safety Code.

Prestressed concrete poles are often used in coastal areas subject to atmospheric corrosion (one manufacturer claims that "concrete poles are foremost in appearance, durability and maintenance-free service").

A riser is an electrical connection attached to the side of a pole; hence, a pole supporting a riser is called a riser pole.   The riser provides a connection between aerial conductors and underground conductors.   The riser conductors are usually protected by conduit, although a U-guard is sometimes used.

This particular concrete pole supports:

• Static wire.
• A three-phase wye-connected (four-conductor) transmission circuit.
• A three-phase primary distribution circuit, tapped to feed, via the riser, an underground circuit to a nearby large customer such as a school, hospital, or shopping center.
• A streetlight.
• A single-phase secondary distribution circuit (fed from a transformer offscreen to the right) whose sole purpose is to provide power for the streetlight.
• Strand-supported CATV cable.

• Three-phase primary electric distribution.
• Transformer.
• Single-phase secondary electric distribution.
• Strand-supported CATV cable.
• Pole-mounted CATV power supply.
• Pole-mounted electric meter to provide power for the CATV power supply.

CATV networks (also calledbroadband networks) incorporate broadband amplifiers spaced throughout the network at intervals of about 2500 feet.   These amplifiers require operating power; this power (at 60, 75, or 90 volts RMS) is provided by a CATV power supply similar to the one shown here, and delivered to the amplifiers over the CATV network itself.   Each power supply is capable of powering a group of 10 to 20 amplifiers located within a radius of a mile or so.   Each amplifier incorporates a DC power pack that rectifies this voltage and provides DC operating voltages for the amplifier circuitry.

The CATV power supply receives its operating power from the power company's secondary distribution circuit, at 115 volts, 60 Hz.   Depending on the power company's billing policies, CATV power supplies may be metered, or they may be billed on a flat-rate basis.   The power supply illustrated here is metered; the meter is visible below the power supply.

The following sketch illustrates a typical wiring diagram for a metered CATV (broadband network) power supply:

• Three-phase open-wire primary electric distribution.
• Single-phase triplex secondary electric distribution.
• CATV cable for public cable TV network.
• CATV cable for Institutional network ("I-Net"), a closed circuit coaxial network for schools and municipal government.
• Multipair telephone cable.
• Two CATV power supplies, one for the public CATV network and one for the I-Net.
• Electric meter (behind the sign) to meter the power used by the two cable TV power supplies.
• Flashing amber warning light.
• Disconnect switch for the unmetered flashing amber light.

The I-Net is physically identical to the public CATV network (and, in fact, it was built by, and is probably still owned by, the CATV company).   However, it is connected only to governmental and quasi-governmental buildings such as City Hall and schools.

Furthermore, there are no telephone or other non-broadband facilities on this pole.   It's a cable-only pole.   Or just cable pole.

Items on this pole include:

• Strand.
• Amplifier.   The amplifier housing (see photo below) contains separate amplifier modules to amplify the trunk signal and to provide outputs to drive the feeder cables.   The amplifier housing also includes a power pack to accept power from the cable TV power supply and provide DC voltage to drive the amplifier modules.
• Tap.   A tap, installed in the feeder cable provides connections for drop cables to individual buildings.
• Fiber optic cable.   Fiber is used to transport broadband signals over distances of several miles, typically 20 to 50 miles.   Fiber cables are typically identified by red plastic identification tags bearing the name of the owner.   Fiber owned by CATV companies is usually lashed to the same strand that supports the coaxial trunk cable; at this particular pole, the fiber bypasses the amplifier to feed other portions of the network.
• Coaxial trunk cable.   Trunk is used to transport broadband signals over distances of a few miles, typically not more than than two or three, and almost never more than ten.
• Coaxial feeder cable.   Feeder is used to transport signals over distances of a few city blocks.   Taps are installed in the feeder at intervals of every other lot or so, to provide connections for drops to individual buildings.
• Disconnected drop.   Drop cables are used to connect taps to individual buildings.   An active drop is connected to the tap; a disconnected drop (sometimes called dead drop) is disconnected from the tap and secured so that it cannot be reconnected except by a company employee.

The Transcontinental Railroad was built by two companies, the Union Pacific (building west from Omaha) and the Central Pacific (building east from Sacramento).   The two companies met at Promontory Summit, Utah, north of the Great Salt Lake, in May, 1869.

The site is now known as the Golden Spike National Historic Site, operated by the National Park Service with the assistance of volunteers who act as docents, exhibit-makers, locomotive engineers, and locomotive firemen.

On May 10, 1869, a ceremony marked the completion of the railroad.   Tradition has it that a golden spike was driven into the last tie to mark the occasion.   Actually, there were four spikes, and the "last tie" (a polished laurel beam) was predrilled to accept them.   After the ceremony, the original last tie was placed on display in San Francisco, where it remained until it was destroyed in the 1906 earthquake and fire.   A replica is now on display in the GSNHS visitors center.