Electromagnetic Pulse (EMP) is generated from any atomic bomb, as far as I know. An EMP could destroy the electronics of many devices, including lighting, electric motors, radios, phones, etc.

So why haven't we read any about that at Hiroshima and Nagasaki? What was the range of disabled electronics? How long before radio or telegraph communications was restored? Were things repairable or else how long did it take to replace them?

Or is there something wrong in my assumptions?

  • After strike on Hiroshima the japan authorities had lost all the communication with the city. In first hours they had no idea what's happening and had to send the scout plane to find out.
    – Matt
    Oct 29 '15 at 9:32
  • @user4419802 I remember reading that on Wikipedia, but it didn't say if that loss of comms was because the bomb blew them up or because of an EMP effect.
    – DrZ214
    Oct 29 '15 at 9:49
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    @user4419802 That is not true at all. Within minutes of the bombing, operators in the bunker beneath the Chugoku Military District HQ notified the Western and the Shikoku Military District HQs by telephone. The Kure Naval District reported Hiroshima's destruction to Imperial General Headquarters by about 8:30, less than 20 minutes after the bombing.
    – Semaphore
    Oct 29 '15 at 9:58
  • One should not forget that the effect of the bombings were barely understood at that time and the destruction was documented much less effectively than in an experimental setup. In middle of the war, a firestrom destroying a whole city and killing thousands of people - no one really cared if any electric device was malfunctioning. Research after the war could evaluate most part of the damage, but any EMP would have minor impact and draw small attention compared to e.g health consequences of radiation damage.
    – Greg
    Feb 9 '16 at 15:14
  • @Greg I can agree with your first sentence, but not the rest. If all or most of the surrounding radios/telegraphs were immediately malfunctioning after the bomb hit, they would've put 2 and 2 together and they would care, precisely because it was "in the middle" of war and they needed to call for help. This would be especially true of Hiroshima because they didn't surrender right afterwards. But apparently the bombs' EMPs weren't strong enough to damage the radios or anything else on the outskirts of Hiroshima or Nagasaki.
    – DrZ214
    Feb 11 '16 at 6:56

Little Boy detonated at ~580 metres above Hiroshima, and Fat Man at ~500 metres above Nagasaki. While all nuclear explosions generate electromagnetic pulses of some sort, at these low altitudes their strength rapidly diminishes with distance, giving them a rather limited area of effect.

The effects of EMP from a surface or low-altitude nuclear burst will extend about as far as the other weapon effects.

- Croddy, E., J. Wirtz, and J. A. Larsen. Weapons of Mass Destruction. Technology and History. 2005.

In fact, most of the the electronics that would have been affected by low altitude nuclear EMPs would have been destroyed by the explosion itself. Thus, you don't hear much about EMP at Hiroshima and Nagasaki because it was largely inconsequential compared to the heat and blast.

The really damaging nuclear EMP comes from nuclear detonations in the stratosphere, as a result of the Compton effect. This is why weaponising EMP generally goes for detonation at over 20km.

  • 10
    Not to mention that the effect of an EMP on any particular piece of electronics is roughly inverse to the size of the electronics - the sheer monstrous size of WWII electronics provided great protection from EMP effects - while our modern nanometre electronics are ultra-sensitive to the same magnitude of pulse. Oct 29 '15 at 22:08
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    And of course, Shockley et al didn't even invent the transistor until 1947, two years after both Hiroshima and Nagasaki. Diodes were in very limited use, and were the size of pencil erasers in 1945 Oct 29 '15 at 22:10
  • The reality is you don't want to detonate a nuclear bomb at 20km because you'll create fallout in a massive area, basically screwing yourself over in unimaginable ways.
    – Nelson
    Oct 30 '15 at 2:53
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    @Nelson Yes and no. A high altitude detonation means far less fallout but over a wider area. A nuclear explosion produces some fallout directly, fission products and un-fissioned material, but far more mass of fallout is irradiated dust and debris kicked up from a ground burst. However, the higher the altitude the further disbursed the fallout will be. So yes, a 20km blast will eventually spread fallout everywhere, but in amounts of much less than a ground burst.
    – Schwern
    Oct 30 '15 at 5:21

My understanding is that the Electromagnetic Pulse induced by a nuclear weapon is mainly due to the ionizing effect of the gamma rays released by the nuclear reaction. However, for this ionizing effect to produce a downward blast of electrons moving at relativistic speed to the ground (the cause of the voltage shock on the ground), the ionizing effect has to take place in a conducting medium.

The atmosphere is a poor conducting medium below 10km so any nuclear explosion below this altitude only provokes a mild shock. Little Boy detonated at about 580m above the ground and Fat Man at about 500m above the ground.

Beside altitude, the most significant factor determining the strength of the EMP is a the gamma rays yield, which is about 0,5% of the total energy yield of the detonation (according to Wiki). Little Boy had a total yield of of 16 kilotons of TNT. As you can see in the chart below, a 0,08 kiloton detonation yields a mild voltage shock even at high altitude.

Graph of the intensity of the voltage shock in terms of yield and altitude

Add to that the fact that electronics in 1945 was vacuum-tube based (transistors would only be developed in 1947) and one can safely assume that the effect of the EMP was negligible (especially compared to the direct thermal effect of the detonation).

  • Thanks but I'm having trouble interpreting the graph. There appear to be multiple labels for the same color. But am I to understand that higher is always better? Even up to 1000 or 2000 km? If so, that would seem ideal because the higher the explosion, the more surface area of Earth is visible and thus more area to damage by EMP.
    – DrZ214
    Oct 29 '15 at 9:56
  • @DrZ214 "Am I to understand that higher is always better?" By better, you mean more destructive, right? Well, the graph shows that the answer is "it's complicated." Nevertheless, the general principle is, lower is more destructive for weak gamma yield and higher is more destructive for strong gamma yield. But there are many complicating factors. I don't know the answer for a a 1000km-high detonation or more but you shouldn't extrapolate. As for multiple labels, the way I see it red is 300km, orange is 200, purple is 60 and green and blue are 100 with different hypotheses.
    – Olivier
    Oct 29 '15 at 10:12
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    @DrZ214 there is only 1 label per colour, but you have to follow the lines carefully. As to "is higher always better". No. It's not. Remember that there's also attenuation on the way down and a higher altitude burst will affect a larger area so the radiation density at ground level decreases. Which means there's an optimal altitude you could calculate based on the curves for both total output as a percentage of yield and radiation density at ground level.
    – jwenting
    Oct 29 '15 at 12:11
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    +1 re: vacuum tube electronics, though they are not invulnerable either (semiconductor detector diodes were in use by 1945, and the kinds used back then were rather less tough compared to modern types. Capacitor dielectrics can be easily damaged by momentary overvoltage, but electronic circuitry back then was often using supply voltages of 90-400 Volts and built of components that could stand operating continously at such voltages. Electrodes in a vacuum can not stand an infinitely high voltage since eventually metal ions will be forced out of the material). And most were designed repairable! Oct 29 '15 at 14:18

The existing answers do a good job of explaining why the EMP effect radius was not as large, but there's another important aspect we need to consider: We're talking about 1945.

EMPs do affect anything electronic to some extent, but they primarily affect sensitive electronics, especially miniaturized electronics like what we use today. The voltages that are perfectly normal for an incandescent light bulb, most electric motors, or an analog telephone will utterly destroy most modern computer equipment. However, that computer equipment largely didn't exist in 1945. If you dropped the same bombs on a modern city, you would most likely see much more EMP-related destruction because sensitive computing devices are ubiquitous today and much of our critical infrastructure depends on their operation.


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