A goal of miners is to find the hash of all possible combinations by the brute force method. This hash corresponds to the private key and gives the access to new operations. Thus, the miner gets a minimum of 25 coins at one time when solving the task. At once set of devices is involved in the calculation of the hash at one time. As soon as someone “guesses” a combination, a new unit opens, and it all starts again.
The mathematical principle is as follows. Devices calculate a hash, which should include a part of the previous block, a random number and a total amount of hashes during the last few minutes. Miners change the random number only as long as it will meet all the requirements of the system.
In fact, coin miners are like those who distribute the data on torrent trackers. We all know that they are the backbone of the p2p network. Thanks to them, each user can download the music or a movie. When loading a user increases his rank, which gives him the opportunity next time to continue to download the data. In the case of the bitcoin the “miners” guarantee the normal operation of the monetary system and carry out a huge number of transactions. Only for their labour they receive no the rank, but a very real monetary compensation.
The mining is carried out on the increasingly powerful hardware hence the complexity of the mining rises. If originally a usual home computer was enough for “miner”, and then you could profitably work at top gaming graphics cards. But today this is not enough – you have to use special devices. Their main task is to quickly calculate the hash while consuming the least amount of the energy.
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 The manufacturer of the special mining hardware or a video card. In general case the same parameters of the equipment from two different manufacturers will behave differently.
 The slot in which equipment connects to the motherboard. Surprisingly, sometimes the hashrate can drop or rise significantly even from the reconnecting to the same slot.
 The algorithm with which you are working. The different hardware with a different speed runs with existing algorithms. For example, at the certain amount of the computation the performance of the Script algorithm will be different from the performance of the SHA.
Also note that some types of a mining hardware and powerful graphics cards can not work with some cryptocurrency or to work with too low speeds.
Of course, manufacturers of such equipment always provide the stated capacity. However, you have not to blindly believe such figures each individual unit will have its own indicator, depending on the factors described above. The surest way to know hashrate is to try out the gear and measure the average, as this value is not constant and can slightly fluctuate.