Let me preface this by saying that I am in no way claiming to be an EMP expert.
OTOH, I have read quite a bit about it, and there are ample references available to the public.
While Bill did not ask how to build a watch, allow me to elaborate.
There are essentially three non-nuclear effects of the EMP, sort of like a combination of punches, as it were.
This is a better technical explanation than I can provide.
Quote:
FIRST EMP COMPONENT (E1)
The first component is a free-field energy pulse with a rise-time measured in the range of a fraction of a billionth to a few billionths of a second. It is the “electromagnetic shock” that disrupts or damages electronics-based control systems, sensors, communication systems, protective systems, computers, and similar devices. Its damage or functional disruption occurs essentially simultaneously over a very large area.
SECOND EMP COMPONENT (E2)
The middle-time component covers roughly the same geographic area as the first component and is similar to lightning in its time-dependence, but is far more geographically widespread in its character and somewhat lower in amplitude. In general, it would not be an issue for critical infrastructure systems since they have existing protective measures for defense against occasional lightning strikes. The most significant risk is synergistic, because the E2 component follows a small fraction of a second after the first component’s insult, which has the ability to impair or destroy many protective and control features. The energy associated with the second component thus may be allowed to pass into and damage systems.
THIRD EMP COMPONENT (E3)
The final major component of EMP is a subsequent, slower-rising, longer-duration pulse that creates disruptive currents in long electricity transmission lines, resulting in damage to electrical supply and distribution systems connected to such lines (Figure 3). The sequence of E1, E2, and then E3 components of EMP is important because each can cause damage, and the later damage can be increased as a result of the earlier damage. In the example depicted in Figures 2 and 3, about 70% of the total electrical power load of the United States is within the region exposed to the EMP event.
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You are not going to beat all of these effects with surge protectors or lightning arrestors.
The electrical grid in the affected area is going to die and will be many months before being restored, if ever.
Any device with significant electronics which is plugged in will be terminally fried, and will be unrecoverable without replacement of the electronics.
Any electronics which are not protected, hardened, or pre-transistor are going to be irreparably damaged and will not be repaired without protected replacement parts.
That would include all wireless and cordless comms, TVs, radios, computers, industrial controls, electrical generation and distribution, etc.
Also, all cars, trucks, trains, etc. built since 1980 or so will be dead. Your car runs off a computer and electronic controls now. All cars are fuel injected and have been for more than a decade. You will need repair parts, and those in parts stores, wrecking yards, etc., are not protected either.
This means a near total disruption in the affected area of electric power; petroleum and natural gas refinement and distribution; telecommunications; banking; water and sewer systems; medical care; air, water, and ground transportation systems; and food distribution.
The burned out systems will have to be replaced, and in many cases, the key components are custom made with a lead and build time of many months. Furthermore, the industry is set up to build a few per month, not the thousands that will need to be replaced. Then you have to move them around and install them in a country with a seriously degraded infrastructure and potential lawlessness. That would be a significant challenge.
If this were to happen, and most nuclear capable countries have the ability to do so, that which is not protected/hardened or in your possession with sufficient fuel and repair parts is all you are going to have to live on for a year or more. I do not think the government is going to be in any position to help most Americans. Neighbors will have to bond together.
Frankly, I expect the lawlessness and misconduct of large groups of the underclass to do as much or more damasge than the initial EMP. They will overwhelm law enforcement and commence rioting, burning, looting, etc. within 72 hours of the event. Since the narcotic supply will quickly be exhausted, followed by the alcohol stocks, eventually followed by the commercially available food supply, every city will quickly look like Katrina, and then go downhill from there.
Those in rural areas will fare better than those in suburban areas, and they better than those in urban areas.
The problem with relief efforts will be that the system is designed to send help from non-affected areas to the affected ones. When a sufficient percentage of the areas are affected, there is little or no one to send help from.
This potential tragedy could be averted and the major components of our lives protected by about a 5% additional cost, with component hardening mandated by Federal law, but they are not.
Sorry for the diversion, I now return you to the discussion of how to construct an EMP protective box. I can think of a lot of things that would be more useful to me after an EMP apocalypse than a laptop, but that was the original question. It might be worthwhile to take an old one that is being replaced, wrap it in plastic and cover that with aluminum foil and or screen wire before putting it in a grounded ammo can, but you would still have to take it out every few months to recharge it.
Hope that helps someone.
TR