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?
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.
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.
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).
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.
