In the Henderson’s Boys book Scorched earth, the Nazis controlling Paris shut down electricity to prevent resistance members from listening to British radio broadcasts. Was this a real German practice or was it simple embellishment on the part of the author?

I'm not aware of any instances when the Germans turned off power to an area or city to stop the inhabitants from listening to broadcast radio such as the BBC. The BBC broadcasted almost constantly to occupied Europe, and the Germans couldn't keep the power off indefinitely. Turning off the power to the entire city of Paris would be essentially pointless. If you let me know the page I can take a look and give a more exact answer on this. However, the Germans did cause temporary localised power cuts for the purpose of smoking out Allied wireless operators and their sets.

A good line of communication between the UK and resistance networks and SOE agents in the field was essential. Without good communications it was impossible to co-ordinate attacks, pass information back, or make agents in France aware of incoming supplies or new agents. While there existed a number of methods outside of wireless transmission, these were incredibly slow. Coded letters using the regular post to agents in neutral countries took days to arrive and then had to be sent back to the UK. Messages carried across the Pyrenees by escape lines faced a similar delay, and messages passed back and forth through amphibious landings were prone to disruption given the inherent dangers of covert landings under the German's noses.

SOE had great success broadcasting pre-arranged phrases on the BBC. If the listening agents heard a certain phrase then they would know that a supply drop was coming on a certain night in a certain location or that an escaping prisoner of war was due to arrive in a certain city. These messages were completely undecipherable to any other listeners. However, while instant, this line of communication was only one way. The agents in France had no way of re-arranging drops or warning Britain that something was wrong. From 1942, some circuits had 'S-Phones' available which allowed them to speak to planes flying overhead equipped with a similar device. However, every second spent talking increased the risk of the Germans noticing a plane flying around the area and shooting it down.

Wireless communication was key, and the Germans knew this. Circuits in occupied France with a wireless operator were able to better co-ordinate supplies and missions as well as pass back time sensitive information. The only delay was the time needed to encode and decode morse messages. By 1944, there were over 150 wireless operators operating in occupied France.

Recognising this, the German Funkabwehr was established in 1940 and oversaw all operations to try and locate broadcasting wireless sets. Their vans were extraordinarily efficient. In most large towns, any broadcast longer than half an hour would see a van full of Gestapo agents turn up on the doorstep. In the summer of 1942 in Lyon, for example, there were eighty detector vans operating.The life expectancy of a wireless operator in the darkest days of the war was just three months.

One trick the Germans used in order to narrow down the location, the Germans would switch off power district by district. When the broadcast stopped, the Germans would know which district the operator was broadcasting from, and be able to converge on their location. The only solutions to this problem were to broadcast from the countryside or use an 'accumulator' instead of mains power. Virgnia Hall, operating in the Haute-Loire in 1944, used a borrowed car battery which was recharged by a resistance member on a bicycle. The best way to avoid getting caught was to exchanges short.

If a wireless operator was caught, then they were often able to warn Britain that they had been compromised. Every person who uses Morse code has a certain 'fingerprint' - the way they type is subtly different. If the operator was replaced by a German, the home station would be able to tell that the style had changed. However, this was sometimes difficult to tell due to poor signal quality. Another method was the use of security checks. An operator might include a series of letters at the start of a message, or spell a certain word wrong. If they were captured and forced to continue broadcasting, they would omit these checks, thereby informing the listener that they were compromised. However, the Germans used these as well for their agents (although all of their agents were actually compromised, a fact they never found out) and so would force the operator to reveal their security checks under torture. To guard against this, operators would be given two sets of checks, one that they could reveal under torture and one to keep secret. The only danger was if an operator or resistance member defected to the enemy willingly, as was the case in 1941 with the Interallie network. Mathilde Carré agreed to cooperate in exchange for her life, and was able to set up an ambush of agents being put ashore. Two SOE agents and a uniformed Naval officer were captured.

In summary, the Germans would never shut down power to the entirety of Paris to stop them listening to the BBC, as this would be needlessly disruptive especially to solve a problem which wasn't very high on their list of priorities. However, in order to smoke out wireless operators working for the resistance or SOE they would often carry out localised power cuts to narrow down their location.