I started thinking about it when I came up with a puzzle which is probably way too elementary for this site, but it is hard for me to understand clearly.
I just bought this small scale thing 
and I wondered – what if I hang one on another one and then hang a 60 kg weight on the lowest one.
Instinctively I get two solutions – 60 kg would be shown on the displays on both scales or 30 kg on both (not caring about the weight of the machine itself).
The top one should show 60 kg, since such weight is hanging on it. The one below too.
But imagine you would stand on a ground and pull down with just a power of 60 Newtons both springs.
My thought process looks like this: To show 6 kg on one device I need to pull with power of 60 Newtons on the spring. When I add an additional spring (not parallel, but sequentially) I am now pulling another spring. I should need twice as much power to pull two springs instead of one in such a way, that the machines would both show I am pulling with the strength of 60 newtons. No? I am sure this works with connecting the machines in parallel. But I am not sure with the sequential alternative.
We are overcoming a resistance of spring to make a change that is shown on display. The second spring should be stealing the difference from the first spring.
Please enlighten me what is the solution and if it is the double 60 kg solution show me where I am going wrong in my thinking.
P.S.: We had a bet with my GF and we need to know who won.
Best Answer
If the two devices are sharing the load, they will each show approximately half the weight. But if you hang one below the other, then each feels the full load. Think about it this way: the one nearest the suitcase "feels" the suitcase. It doesn't care whether you are holding it with your hand, attached to a spring, or whether it is dangling from the side of Mount Rushmore.
The top spring is measuring the weight of "the thing below it" - that is the suitcase plus the other spring gage. It doesn't care whether the spring gage is in the suitcase, or attached to the top.
If you were right, be nice about it to your girlfriend... I don't want to cause trouble.
PS - I think your confusion comes about from the difference between force and displacement. When you have two springs in series, then indeed you need to pull twice as far in order to get the same force registered. That is because the spring constant of the combination is half as big. But each device, since it registers half of the total displacement, registers the same force as before.